I just signed up here myself, and the Steam questions were kind of weird, in that the first one asked for my steam username (which can't actually be used to find me on steam AFAIK), and didn't mention that it was optional.
I did find out how to get the profile ID# despite having a custom profile url, though. Adding ?xml=1 to the end of the profile URL returns an xml file instead of the webpage, and the profile ID# is the number enclosed by <steamID64> and its closing tag.