Pass the field value in the context and prefix the field name with default_ so the field in the payment register wizard will be automatically set for you.

In the following example, we suppose you have a reference (char field) in the account move model and you want to pass its value to the payment wizard, we take into consideration that the action_register_payment method is multi-record (self could be a recordset with multiple account moves)

Regards,
J Wick