If you forgot your user or account name on your Mac
When you set up your Mac, you entered your full name and created an account name. The account name is sometimes called a short name because it’s usually a shorter version of your full name or a name you created. When asked for your user name—for example, in the login window or when connecting for file sharing or screen sharing—you can enter either your full name or your account name. The account name is used by your Mac to keep track of all your files and information. Because of this, the account name is permanent; you cannot change it without creating a completely new user.
If you can’t remember your account name, open a Finder window and choose Go > Home. Your account name appears next to the house icon at the top of the Finder window.