So here's the Firebase Initialization

@InitApi
fun initializeFirebase(ctx: InitApiContext) {
val context = Application().applicationContext
FirebasePlatform.initializeFirebasePlatform(object : FirebasePlatform() {
 val storage = mutableMapOf<String, String>()
 override fun store(key: String, value: String) = storage.set(key, value)
 override fun retrieve(key: String) = storage[key]
 override fun clear(key: String) {
 storage.remove(key)
 }
 override fun log(msg: String) = println(msg)
})
val firebaseConfig = FirebaseOptions(
 apiKey = "AIzaSyB_x6qG-gIkUSQSYw4Op5D7lSgjP2xa9G8",
 authDomain = "ticketsystem-66e93.firebaseapp.com",
 projectId = "ticketsystem-66e93",
 storageBucket = "ticketsystem-66e93.appspot.com",
 gcmSenderId = "180179590377",
 applicationId = "1:180179590377:web:ebf237ea60ab9b72132478"
)
val firebaseApp = Firebase.initialize(context = context, options = firebaseConfig)
ctx.data.add(firebaseApp)

}

Here's a test to see if it has connected by querying Firebase Auth

@Api("testFirebaseConnection")
suspend fun testFirebaseConnection(ctx: ApiContext) {
/*try {
 val result = ctx.data.get<FirebaseApp>()
 ctx.res.setBodyText(result.toString())
} catch (e: Exception) {
 ctx.res.setBodyText("error")
}*/
val auth: FirebaseAuth? = try {
 Firebase.auth
} catch (e: Exception) {
 null
}
if (auth != null) {
 ctx.res.setBodyText("Connected to Firebase successfully. $auth")
} else {
 ctx.res.setBodyText("Failed to connect to Firebase.")
}

}

And here's me trying to create a user with Email & Passowrd

@Api("emailPassword")
suspend fun signIn(ctx: ApiContext) {
try {
 val auth = Firebase.auth
 val userCredential = auth.createUserWithEmailAndPassword(
 email = "------@gmail.com",
 password = "Password_123"
 )
 val user = userCredential.user
 if (user != null) {
 ctx.res.setBodyText("Signed in with Email & Password. User: ${user.uid}")
 } else {
 ctx.res.setBodyText("Failed to sign in with Email & Password. User is null.")
 }
} catch (e: Exception) {
 ctx.res.setBodyText("Failed to sign in with Email & Password. Error: ${e.message}")
 // Log the full exception stack trace for debugging
 println("Error signing in with Email & Password: $e")
}

}

The app connects to Firebase but there's a 500 error when I call the createUserWithEmailAndPassword API.

CoroutineExceptionHandlerImpl.kt:7
ResponseException: URL = http://localhost:8080/api/emailPassword, Status = 500, Status Text = Internal Server Error

kotlin.NotImplementedError: An operation is not implemented.
at com.google.firebase.auth.FirebaseAuth.createUserWithEmailAndPassword(FirebaseAuth.kt:429)
at dev.gitlive.firebase.auth.FirebaseAuth.createUserWithEmailAndPassword(auth.kt:58)
at org.example.ticketv2.api.Firebase_FunctionsKt.signIn(Firebase Functions.kt:93)

propagateExceptionFinalResort @ CoroutineExceptionHandlerImpl.kt:7
handleUncaughtCoroutineException @ CoroutineExceptionHa...erImpl.common.kt:48
handleCoroutineException @ CoroutineExceptionHandler.kt:28
protoOf.handleJobException_9fdet1_k$ @ Builders.common.kt:190
finalizeFinishingState @ JobSupport.kt:228
tryMakeCompletingSlowPath @ JobSupport.kt:907
tryMakeCompleting @ JobSupport.kt:864
protoOf.makeCompletingOnce_b13xy2_k$ @ JobSupport.kt:829
protoOf.resumeWith_dtxwbr_k$ @ AbstractCoroutine.kt:97
protoOf.resumeWith_b9cu3x_k$ @ Continuation.kt:54
protoOf.resumeWith_dtxwbr_k$ @ kotlin-kotlin-stdlib.js:5916
protoOf.run_mvkpxh_k$ @ Continuation.kt:54
eval @ JsMicrotasksDispatcher.kt:11
Promise.then
protoOf.dispatch_qa3n0o_k$ @ JsMicrotasksDispatcher.kt:11
dispatch @ DispatchedTask.kt:157
dispatchResume @ CancellableContinuationImpl.kt:470
resumeImpl @ CancellableContinuationImpl.kt:504
resumeImpl$default @ kotlinx-coroutines-core.js:1113
protoOf.resumeWith_dtxwbr_k$ @ CancellableContinuationImpl.kt:364
protoOf.invoke_py2q9a_k$ @ Continuation.kt:54
protoOf.invoke @ kotlinx-coroutines-core.js:4236
notifyCompletion @ JobSupport.kt:367
completeStateFinalization @ JobSupport.kt:322
finalizeFinishingState @ JobSupport.kt:239
tryMakeCompletingSlowPath @ JobSupport.kt:907
tryMakeCompleting @ JobSupport.kt:864
protoOf.makeCompleting_2ycklh_k$ @ JobSupport.kt:807
protoOf.completeExceptionally_xyzekf_k$ @ CompletableDeferred.kt:89
eval @ Fetch.kt:121
protoOf.doResume_5yljmg_k$ @ Fetch.kt:46
protoOf.resumeWith_b9cu3x_k$ @ Standard.kt:55
protoOf.resumeWith_dtxwbr_k$ @ kotlin-kotlin-stdlib.js:5916
protoOf.run_mvkpxh_k$ @ Continuation.kt:45
protoOf.process_myqcf5_k$ @ JSDispatcher.kt:127
eval @ JSDispatcher.kt:55
Promise.then
protoOf.schedule_o777if_k$ @ JSDispatcher.kt:55
protoOf.enqueue_uiib2n_k$ @ JSDispatcher.kt:118
protoOf.dispatch_qa3n0o_k$ @ JSDispatcher.kt:56
protoOf.resumeWith_dtxwbr_k$ @ DispatchedContinuation.kt:196
protoOf.resumeWith_dtxwbr_k$ @ SafeContinuationJs.kt:34
eval @ Continuation.kt:45
Promise.then
protoOf.doResume_5yljmg_k$ @ Fetch.kt:35
getBodyBytes @ kobweb-frontend-browser-ext.js:921
protoOf.doResume_5yljmg_k$ @ Fetch.kt:46
protoOf.invoke_d9fzmj_k$ @ kobweb-frontend-browser-ext.js:962
l @ kobweb-frontend-browser-ext.js:1011
protoOf.doResume_5yljmg_k$ @ IntrinsicsJs.kt:163
protoOf.resumeWith_b9cu3x_k$ @ Standard.kt:55
protoOf.resumeWith_dtxwbr_k$ @ kotlin-kotlin-stdlib.js:5916
protoOf.run_mvkpxh_k$ @ Continuation.kt:45
protoOf.process_myqcf5_k$ @ JSDispatcher.kt:127
eval @ JSDispatcher.kt:55
Promise.then
protoOf.schedule_o777if_k$ @ JSDispatcher.kt:55
protoOf.enqueue_uiib2n_k$ @ JSDispatcher.kt:118
protoOf.dispatch_qa3n0o_k$ @ JSDispatcher.kt:56
resumeCancellableWith @ DispatchedContinuation.kt:217
startCoroutineCancellable @ Cancellable.kt:26
protoOf.invoke_neaz0o_k$ @ CoroutineStart.kt:88
protoOf.start_rn6v44_k$ @ AbstractCoroutine.kt:123
launch @ Builders.common.kt:52
getBodyBytesAsync @ Fetch.kt:45
eval @ Fetch.kt:120
Promise.then
fetch @ Fetch.kt:115
protoOf.get_89f2h8_k$ @ HttpFetcher.kt:92
protoOf.get_3u5zdv_k$ @ ApiFetcher.kt:81
protoOf.get$default_m5lv9o_k$ @ kobweb-frontend-kobweb-core.js:211
protoOf.doResume_5yljmg_k$ @ Index.kt:84
protoOf.resumeWith_b9cu3x_k$ @ Standard.kt:55
protoOf.resumeWith_dtxwbr_k$ @ kotlin-kotlin-stdlib.js:5916
protoOf.run_mvkpxh_k$ @ Continuation.kt:45
eval @ JsMicrotasksDispatcher.kt:11
Promise.then
protoOf.dispatch_qa3n0o_k$ @ JsMicrotasksDispatcher.kt:11
dispatch @ DispatchedTask.kt:157
dispatchResume @ CancellableContinuationImpl.kt:470
resumeImpl @ CancellableContinuationImpl.kt:504
resumeImpl$default @ kotlinx-coroutines-core.js:1113
protoOf.resumeUndispatched_tyhwz7_k$ @ CancellableContinuationImpl.kt:591
eval @ Standard.kt:71

I've used js
Here's my code

import kotlin.js.Promise
@JsModule("firebase/app")
@JsNonModule
external object FirebaseApp {
 val initializeApp: (config: dynamic) -> dynamic
}
@JsModule("firebase/auth")
@JsNonModule
external object FirebaseAuth {
 val getAuth: () -> dynamic
 val createUserWithEmailAndPassword: (auth: dynamic, email: String, password: String) -> Promise<dynamic>
}
fun createUser(email: String, password: String): Promise<dynamic> {
 val auth = FirebaseAuth.getAuth()
 return FirebaseAuth.createUserWithEmailAndPassword(auth, email, password)
}
fun initFirebase() {
 val firebaseConfig = js(
 """
 {
 apiKey: "-",
 authDomain: "-",
 projectId: "-",
 storageBucket: "--",
 messagingSenderId: "-",
 appId: "-"
 }
 """
 )
 FirebaseApp.initializeApp(firebaseConfig)
}

Then initialize Firebase then create user

 initFirebase()
 createUser("example@gmail.com", "Password_123")
 .then { userCredential: dynamic ->
 val user = userCredential.user
 console.log("User created successfully. User ID: ${user.uid}")
 }
 .catch { error ->
 val errorMessage = error.message
 console.error("Error creating user: $errorMessage")
 }

Here, I use the same js way but realise using Java in the server is better.
You can find my detailed explanation.