How do I [20 M] tell my friend [19 F] I like her even when she has a boyfriend?

So, I'm going to invite this girl to a small kickback tomorrow. We're cool and gotten to be pretty comfortable around each other. I started having a thing for her, and other people noticed it too. I swear sometimes the body language she gives off seem like she likes me.

The thing is that she has a boyfriend, and she talked casually about him before. She probably thinks that since I already know, I must not like her. But I started getting feelings for her, so tomorrow I plan on just telling her straight up that I like her.

Should it be something like "Did you know I like you? I know you have a boyfriend but I just wanted to let you know that, sorry" and just move on? Sorry for the stupid question, just want some guidance and hopefully not make things awkward.

tl;dr: How do I tell a my friend I like her even when she has a boyfriend?

EDIT: Alright, I have decided not to tell her. It is interesting how a different mindset can change our conversations. Our facebook messages are really chill right now since I'm not just thinking about "getting it in"...I just feel more 'loose' and less 'tense' if that makes any sense. Thanks guys. I don't want to ruin any relationships. I will move the fuck on with life

EDIT2: she just randomly messaged me saying "is it unusual we get along?" the fucking feels.