How can I get my follower out of power armor?

I'm certain that this has been answered before, but I can't find the question right now.

Basically you just speak to them and there is a chat option to ask them to get out of the armour. It should replace the 'Relationship' option, under 'Talk'.

Just a warning. You can't get Preston Garvey out of power armor if you haven't finished retaking the castle after unlocking it

I talked to Piper in order for her to exit the power armor that i found, but she would just start talking about friendship, and I couldn't seem to get her out. So I figured out another way method: find another power armor your companion can get in, and order them to enter that one. This will essentially make them get out of their current power armor, leaving it available for taking back. Make sure your companion doesn't get into the second power armor when you do this.