[x sanitize_url=0 keepjs=1]The plugin redirects to the appropriate page after payment like this:
Your approach is unusual. Normally, developers would use the Formidable API add-on or one of the payment processing add-ons so you can use their hooks to further process actions. The confirmation action is executed, all earlier trigger actions would have been completed and the entry would have been saved, ending the submit process.
Unless, there's a WordPress hook you can use in the RedSys plugin, I have no idea how you would run an additional trigger action to register users after the confirmation message appears.
One possibility is to register the user before payment with the Formidable User Registration add-on. Set the registration add-on to require admin approval before account creation. The user account won't be created until the admin approves the entry, which can happen after payment confirmation is received. See this:
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