lloyd094 that's not possible natively, however there are some solutions if you write new code.
The difficulty is that the search endpoint returns discussions, not posts, so we cannot have an additional filter or gambit in the search that affects which posts are searched. The actual text search is provided by the FulltextGambit which always looks inside all posts of the discussion.
Your solution would be to implement a new search gambit that takes both a text and a post number, or have the post number hard-coded. Then you have two solutions: make that a new special search gambit with a prefix, for example called textinpost:1,"hello", or make it a full text search gambit and replace the original full text gambit. The first solution would provide an additional method if you're using this from an external app using the REST API for example, while the second solution would affect all searches performed from the forum.
You could even have a more advanced full text search gambit that behaves differently for discussions under different tags.
EDIT: there is a tag: gambit that exists natively which solves the other half of the question
