Click on the "Forgot Username" on the tool bar:
Mobile users may need to click this symbol to get a menu to show:
Mobile users then click the "Log In" button and can find the "Forgot Username" there:
It will ask you to enter your email address. Enter it and click "Submit":
You should see a message:
You will then see an email in your inbox:
In this email you will find your site Username that is used for logging in.