He's saying don't add a button. Go into the menu system and set it to free play.

If you don't know how to do it, download the manual here: https://www.ipdb.org/files/4446/Bally_1999_Revenge_From_Mars_Operations_Manual_February_1999_OCR_searchable.pdf

Edit - Sorry missed your post about 'earning' a credit. There are several things you can do to make high scores important - see the next post.

Not sure that a button helps with that feeling you are looking for, but it will only frustrate your casual playing friends and the next owner.

Better option is to put out some coins to use when there are not any credits left. THat way you can also monitor how long it takes to use up a 'roll of quarters'