How to Query Facebook Page Posts for an External Partner?

I have already created a Facebook App and selected the Other option during setup. My goal is to fetch posts from a Facebook Page owned by an external partner (not my own page) using the Facebook Graph API.

Could you give me any guidance how to do it?

Since I don’t own the page, how should the external partner authorize my app so I can obtain a valid Page Access Token?

What is the correct process for them to authenticate my app and grant access to their page?

After authentication, how do I correctly retrieve the Page Access Token to query /page_id/posts?

Are there any additional app configurations or API calls required?

Thanks for any help!