Navigate to the thread of interest. Activate search. Press the gear wheel that presents the extended search drop-down. Replace 'everywhere' with 'this thread'. In the member selection text area, choose your self or other member of interest.
Happy searching with your newly found power.
Thread title suggestion: How do i search a specific thread for.posts of a specific member?
Edit:
Using search, I found similar question posted by
@RainAndSadness.
https://sanctioned-suicide.net/threads/is-it-possible-to-search-a-thread.16549/