In the past, people have said that you can use the "Send Feedback" link for requests like this. But I don't have any personal experience with this. If you use that link, make sure you include enough information to give the developer a resonable chance to help you. I am guessing that you would want to include your full name, your email address, what you think your username is, your birthday, the full names of a couple of the people in your tree.
I don't know if the developer is setup to retrieve accounts like this, but that information will be enough to give them a fighting chance, or maybe to steal your identity.