underline is a typographic effect that underlines a sequence of characters.
Underlining permits to add emphasis on a couple of words but clashes with the conventional rendering of link. Use it when it couldn't be confused for a hyperlink.
You needs to wrap your words with two under characters __.
This text is not underlined while __this one is__
This text is not underlined while this one is
You can also use the underline boolean attribute .
Example:
This text is not underlined while <itext underline>this one is</itext>
This text is not underlined while this one is
This [[?underline=false|link]] is not underlined while [[?underline|this one]] is.