Custom registration form based on subdomain

Hi all,
I need to show custom fields on the registration form based on the subdomain.

I finished coding models for every subdomain and know-how to get to on the registration form.
but now how I can calls fields based on a subdomain?
even “REGISTRATION_EXTENSION_FORM” on lms.env.json just accept one attr?
do I need to make any customization on /edx/app/edxapp/edx-platform/common/djangoapps/student/ on this function get_registration_extension_form?

open-edx- release = hawthorn

appreciate your help