Telling my parents I’m starting HRT (HELP!)

For some context, I’m a 19 year old ftm who does NOT live with my parents, though I do go see them fairly often. They know I’m trans, and my dad doesn’t really give a damn, but my (emotionally abusive) mom is insistent that it’s all a phase, and has been ignoring it since I was twelve ( when I told her). My grandfather also lives with them (though I don’t think he’d really notice)

I’m going to starting HRT the week of December 6th and I don’t know how to tell them.

TL:DR- I want to tell my emotionally abusive mom I’m going on HRT and Idk how.