According to the documentation on using iFrames, the commands need to be nested in within.

I have a contact form embedded in an iFrame that I want to test:

 await within({frame: "#iFrameResizer0"}, () => {
 I.see('Contact Us');
 I.fillField('#edit-message', 'This is a test message.')

I.see works, but I.fillField does not fill in the field.

When I go to the contact form page directly (when the contact form is not embedded in an iFrame) and use

I.fillField('#edit-message', 'This is a test message.')

Then the field is filled in correctly.

So what do I need to do to use fillField inside an iFrame?