How do you type in an underscore on Samsung Edge?

Well, I'm temporarily borrowing a Samsung Edge from a friend and have one problem. I'm trying to sign in to Yahoo! Mobile Messenger, but when I try to put in an underscore, it's nowhere to be found! I've tried the soft key thing, but no symbols come up... I asked around but no one seems to know how to do this... When texting, the underscore can easily be found, but that doesn't seem to be the case for mobile IM... Is it possible that the phone is unable to type in underscores on the Yahoo! Mobile IM?|||uhhh