From Test-Scratch-Wiki

If you forgot your username, you can search for the titles of projects you uploaded, forum topics and studios you created, or projects you commented on, and see if you can recover it from there. If you email the Scratch Team from the address you signed up with, they may be able to find your username. You can also go to the reset password page and enter the email address you registered with, and your username will be emailed to you.
Cookies help us deliver our services. By using our services, you agree to our use of cookies.