How to Find your Mac Username and Password
When you first set up your Mac, you entered your full name and created a username. This is sometimes called a "short name" since it is usually a shorter version of your full name. When asked for your username — for example, in the macOS log-in window or when connecting for file sharing or screen sharing — you can enter either your full name or your username.
The username is used by your Mac to keep track of all your files and information. Because of this, the username is permanent -- it cannot be changed.
If you can’t remember your username, open a Finder window and look for the house icon in the sidebar. The house icon identifies your home folder, which is named with your username.
If you don’t see the house icon, choose Finder → Preferences, then click Sidebar. Under Favorites, select your home folder.
Allow access for all users, or add each individual user you would like to use to log in from Screens. This should be performed in the Sharing preference pane in System Preferences. Configure these settings for each service you plan to use with Screens.