Good Day!
Using payment requests for F111 is similar to using invoices in F110. Maintain customization in F11CU similar to how you setup FBZP. After payment method configuration, you configure TRM and other setups.
Payment requests are seen in F8BT and can be dynamically inputted in F111 runs. You may check PAYRQ* tables for details of payment requests.
Hope this helps.
Jayj