Identity validation before/after operation

I´m designing an extranet module for vendors that want to return and exchange unwanted products (change size or color, return broken products, etc.).

I have to validate the identity of the vendor (via SMS, mail or id card number) before sending the request.

Is it better to validate identity before starting the all process that is an extended form? or is it better to let the user complete the form and validate identity at last...just before sending the request?

Thanks for your time!....