I had major problems finding this out for the launcher and account got locked out as a cause of it.
I went to go reset my login/password, I gave them my associated email. and in the email I could reset my password, but along the entire way, it didn't tell me my square enix user ID.
Please change this asap.
Your ID should be in the password change email.
It's a good thing I remembered it from a long time ago, but it took a few guesses.