I represent a marketing agency.

I want to run ads on Client's Facebook page using my own business manager account (Ad account).

To get access to Client's Page, I have integrated "Facebook Login for Business".

Using this flow, Client is able to share his Page access to my Facebook App. At the end of the flow, I am able to retrieve "Page access token" for Client's page.

But I cannot see this Page in my Business Manager assets. Hence my System User token does not have access to this Page and I get error while creating Adset (mentioning Client's Page) via Marketing API.

How can I add this page in my business manager as assets programmatically? I do not want to ask client to do this offline via his business manager UI, I am assuming this can automatically be done via Fb Login integration.

I have seen some apps that use their ad accounts and the user's Facebook page to run ads. They ask the user to log in via Facebook. How are they doing this?