Authorize.net API Login ID and Transaction Key Setup Merchant Gateway
In order for you to begin processing transactions - you will need to get your API login ID and transaction key. You can call your gateway provider to do so. For Authorize.net, once you have received your credentials, go to account/tools/site settings.
Once there follow the steps below and paste your credentials into the fields highlighted, then click save.
Once this is complete you can create an ecommerce form which will use these default credentials. If you wish to use other credentials on the platform - you may do so by overwriting this default in the individual modules such as shopping cart, subscriptions, etc.