2) Set up a payment profile for your Payment Service Provider (PSP, I use Mollie for reasons mentioned in this thread) and create a (hidden) menu item as described on a.m. page.
3) In the Fabrik form:
build the payment link as described on the a.m. page:
generate a random reference number in a hidden field and use it as the optional argument order_number
also use the silent=1 argument
use the Redirect plugin with the RO Payments link in “Jump page” and “Content Redirect” to New Page
This causes a redirect to RO Payments and thereafter to the PSP after the form has been saved.
4) After the payment has been handled, ROP triggers an event onPaymentComplete: https://rolandd.com/documentation/ro-payments/a-list-of-ro-payment-plugin-triggers
You then have to write your own handler to finish the transaction, you can copy from the examples /plugins/jdideal/ropayments/ ropayments.php
Use order_number to link to the right record in the Fabrik table.