If you get lost you can always contact Keap support via chat or phone and they can help you determine the appropriate merchant ID.

Keap has now made it possible to see your merchant ID account by going to E-Commerce Setup > Orders > and clicking on the Default Merchant Account dropdown.

That should get you a list of all of your merchant accounts set up in Keap and you can choose the applicable merchant account ID. Once you have that ID number, simply enter it into the merchant ID field in Fortapay.