MATL, 9 bytes

&mB!8e!XB

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Takes input as string, then encoder

&m % 1-based index of first input in second
B % convert from decimal to binary. Produces a logical vector
! % transpose
8e % reshape into matrix of columns of length 8, padding end with zeros.
! % transpose
XB % convert from binary to decimal
 % (implicit) convert to string and display

Of note, B returns a matrix with the binary strings as rows, so they are automatically padded to the same length.

MATL, 17 bytes

&m2Gt&mBwY)!8e!XB

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Of note, B returns a matrix with the binary strings as rows, so they are automatically padded to the same length.

MATL, 9 bytes

&mB!8e!XB

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Takes input as string, then encoder

&m % 1-based index of first input in second
B % convert from decimal to binary. Produces a logical vector
! % transpose
8e % reshape into matrix of columns of length 8, padding end with zeros.
! % transpose
XB % convert from binary to decimal
 % (implicit) convert to string and display

MATL, 9 bytes

&mB!8e!XB

Try it online!

Takes input as string, then encoder

&m % 1-based index of first input in second
B % convert from decimal to binary. Produces a logical vector
! % transpose
8e % reshape into matrix of columns of length 8, padding end with zeros.
! % transpose
XB % convert from binary to decimal
 % (implicit) convert to string and display

Of note, B returns a matrix with the binary strings as rows, so they are automatically padded to the same length.