I would rather see this implemented in the __init__ method of the form. There's no reason to include the field when working with a v2 auth or non-multidomain keystone instance, and I'm not a huge fan of having variables defined in the scope of the form class.

Particularly in this case there is likely to come a point when we can introspect the endpoint to determine domain support at runtime, so we will want this to be instance-level, not class-level.