If you don't have a clear idea of your target audience yet, it's worth including the screener questions in your user-research plan. Depending on your product's nature, you may need to specify average users' age, occupation, interests, problems they might face, involvement level, etc.
It's much easier to find suitable participants for an existing product. In this case, you will simply need to recruit some of your app's actual users.
However, if your product hasn't entered the market yet, you should carefully select the research participants. It might be helpful to choose your direct competitors' customers. Thus, you can get an idea of a common service user. Besides, it will help you learn how to make your product stand out.
Interviewing target users gives precious insights that you will later use to create an MVP
or redesign an existing product. No efficient UX solution can be created regardless of user expectations and pain points. That's why you should prepare an accurate list of questions you're going to ask — the basis of your research.