The e-mail address that serves as your username for mon.scarlet can be found on the 3rd page of your bill.
Don't have a bill at hand?
If you no longer know the e-mail address you chose to log in to your mon.scarlet customer zone, please check your mailboxes to see if you have received an activation e-mail from Scarlet. If you find such an e-mail, it means that your e-mail address is your login.
Your login is also included in every e-mail you receive from Scarlet.