How can I link the registration form to the payment form

Hello, you can link first form with the second form by prepopulating the fields of the second form with the values of the first form. This will take some PHP knowledge.

Send-POST-Data-From-JotForm-Using-PHP-in-Custom-Thank-You-Page

 Kindly take a look at the user guide posted above for more information. If you do need further support, do let us know. Thank you.