There is no way to underline but there is away to cut 'n paste or copy 'n paste.
If you do Edit-Copy of some text elsewhere, say in Word or your email, then in the Note screen, press the Ins key while holding the Shift key.
To cut and then paste from and to Notes, highlight the desired text in the Note screen, press the Del key while holding the Shift key to delete and then move to where you wish to copy and do the Shift-Ins thing.
To copy and paste, do as above but do Shift-Ins immediately after and in the spot where you did the Shift-Del thing and then Shift-Ins again where you want a copy of the highlighted text.
This works in most places in Windows whether the menu driven Edit-Copy and Edit-Paste is available or not ...