Apologies for the necromancy, but I found this thread helpful and wanted to report my findings in case it might be helpful to someone else.
I recently restarted a dynasty, and was having major issues getting the "Ask to move in" interaction to appear on the menu. A couple of posts in this thread mentioned being "sociable" - and that was the key for me. When my founder (finally) did an interaction that Agnes found to be "sociable" (rather than okay, amusing, alluring, etc.), the option appeared. The one that worked for me was the friendly hug, under the Friendly menu. The relationship was good friends / romantic interest / girlfriend.