You can add a leaf switch to the credit button and wire one of the coin switches. I would not recommend that you do that - but that's how you do it.
The best way is to set the free game to 10,000 - that way, you will always have a credit on the machine. This is done by the test button on the front door. The maximum number of credits is also set via the dip switches.
Download the manual off IPDB, and it explains how to set the first free game and the dip switch settings.
