HMAC does it matter if I chose the cipher text or the plain text as the message? I am using HMAC to create a message authentication code. My question is, does it matter if I use cipher text + encryption key or plain text + encryption key? I had a look at a implementation and as far as I can see they just concatenate the values (message and encryption key) and hash them.
Yes, it does matter. A MAC should always be over the ciphertext, not the plaintext.

HMAC(key, ciphertext) = Hash((DerivedKey xor OuterPad) || Hash((DerivedKey xor InnerPad) || Ciphertext))

Where || denotes concatenation.
OuterPad is a one-hash-block long hexidecimal constant 0x5C5C...5C.
InnerPad is a one-hash-block long hexidecimal constant 0x3636...36.

Of course you should use an existing implementation of HMAC instead of trying to make your own, but if your library is just concatenating values and then hashing them it's not HMAC, and you should use a different library (assuming the library actually calls it HMAC).

There are other MACs than just HMAC, such as SHA-3's KMAC or Poly1305. They also should only be applied to the ciphertext. Your library may be using one of these. I happen to like the libsodium library. It uses Poly1305 for its MAC. It also includes a design that makes it hard to misuse: it's impossible to use a symmetric cipher without properly applying a MAC, so you don't have to worry about the answer to this question. You just call the appropriate crypto_secretbox function!

The following question and answers give reasoning for Encrypt-Then-MAC being optimal: Should we MAC-then-encrypt or encrypt-then-MAC?