If you want to type the underline tags rather than using the editor, put [U] before the word and [/U] after the word.
You can do the same for bold using [B] [/B] tags and italic using [I] [/I] tags.
And to type these codes without them doing their thing, as I did here, use [NOPARSE] [/NOPARSE] tags (which aren't on any buttons in the editor).