In WordStar you need to block mark an area of underlined text by typing ^PS at either end of the text. This could have been done, but would have added unnecessary complications and screen updates. The WordStar Command Emulator works in a subtly different way, as follows:
To underline a single word, place the cursor within the word and type ^PS. You can also use F8 to mark the start of a larger area, navigate to the end, and type ^PS to underline the whole block. This is a cross between WordStar and Word methods of working. If you change your mind, press ESC to unmark the block.
To toggle underline on/off as you type, type ^PS then the text you want underlined, then ^PS to continue in normal text.
The same applies to ^PB (bold), and ^PY (italics).