Once you have signed up to your Payment Gateway you can log in to your Payment Gateway account, copy your API Key and then head back to your MOBLE Form and paste it into the API Key field. Once you click Save in the top right you form will now be set up to receive online payments.
The Image below shows where to find your API on Stripe. You need to copy the Live Secret Key.
The Image at the bottom of the page shows where to paste the API Key on your MOBLE form.