How to change MFE settings (authn)

I want to change default set of fields in the registration form which, as far as I understand, is a part of “authn” MFE. I learned that by setting ENABLE_DYNAMIC_REGISTRATION_FIELDS to false I can get rid of e.g. “Country/Region” field, by how to set this variable? Shall I use a specific template patch in a plugin? Thanks in advance.