Payers that don't register

Users are prompted to complete registration after the payment per settings. Once in a while the user does not complete registration. Their name, etc gets piped in but there is no username. The username field is not there and so I cannot create a username on user’s behalf.

How can I get user to complete their registration if they did not do so after payment — is there a special url? Or how can I do it for them?

Thanks