Asking a long distance girl out?

So I already know I am going to ask her out, I'm going to her prom next month and I'm going to do it then. But I'm not sure how to ask her out. Normally I would think something like "Would you like to grab dinner sometime." but With the fact of it being a long distance relationship that doesn't very well work. I'm traveling over 450 miles to go to her prom, and I know she wouldn't just ask me so she at least likes me I know that. But how how do you ask somebody out for a long distance relationship..

Thoughts?
Asking a long distance girl out?
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