You usually don't want to do that. You put your search-term IN the magnifying-glass box, select "This thread" (or whatever), and search. Simple example in the attached image.
IF you only wanted term 'simple' BY member 'Jason' then you would use the "By:" option. (Example: "I know TubeLab George talked about snowblowers but where?") Most often I have no idea who said something, but occasionally I do.
And no, despite playing fast and easy with familiar GUI expectations, this software does not seem to have as much Help as a box of Eggos.