How to conduct user testing to identify the optimal position for an entry point of a feature

I'm conducting a user testing session to identify what is the optimal entry point on a flight booking mobile app, to introduce users to its frequent flyer program.

Potential entry points are

• after booking of flight tickets
• before check-in
• on the home screen
• other suggestions by the user

the design itself is a static component with some copy and a 'register now' button.

This question is not looking for the answer of which is the optimal entry point, but more regarding what kind of a test should be presented to the participant. Normally user tests are conducted to get feedback on a particular feature. however, the aim of this is to identify the best place in the app (and users journey) to include an entry point to sign up.

Would appreciate suggestions and feedback on this