Your User ID is not your username, it's part of the backend of things and appears in your URL when other people visit you in browser play.
If you don't know your User ID, it can be found in the /mkg/ spreadsheet Fairy Doll tab (If you're on it) or give someone who uses browser play your master name and they'll get it for you.

User ID: Fairy:
Recommendations: Picture size: Picture type: