forgot my administrator username

macOS Sierra 10.12 and macOS High Sierra 10.13

Shutdown the computer.

Start the computer, then press and hold down command and R keys to start from macOS Recovery.

When you see the Apple logo and the progress bar, release the keys.

You have to press hard the Trackpad to register the click.

Select the “Language”, if not already.

Click the arrow button at the bottom.

Wait until OS X Utilities window shows up.

Move the mouse pointer to the menubar at the very top of the screen, click "Utilities” and then select "Terminal"

from the dropdown.

Terminal window will appear.

Type in resetpassword and press enter on the keyboard.

Reset Password window will open reveling all user names.

Note down the username.

Quit Terminal.

Quit macOS utilities. Select startup disk.