This post and your follow up is confusing. You say you don’t want the bottom bunk, but you ask how to convince roommate to take top bunk. Which is it?
ETA: If you are saying that there is a bunk bed and one loft, and you want the loft, you can ask for it but be prepared to compromise, i.e., everyone switches beds at some point in the year so it’s fair to all, assuming that everyone will prefer the loft.