How to add custom fields for specific domains?

[alfalogic]

I actually don't know how to organize this thing. For some domains, like .RU/.SU the registrar asks for Passport Number and additional details (I use EMAIL as registrar in my WHMCS).

 

Can someone please suggest me any way to add a custom field for users who order any of those specific domains or any other way that will request the user to enter passport number IF THE ORDER IS FOR .RU or .SU domains?

 

Thanks for your support.