Type an "underscore" character ("_") to glue together the different parts of your Username.
In order to type an underscore, you must hold down the Shift key and then press the hyphen (or dash) key once, and then release the Shift key.
The underscore character lines up with the bottom of words. Be careful you don't type a hyphen instead.
Some people think of the "underscore" character as an underlined blank space.