I see Mailchimp has a feature to create facebook campaigns through its UI and I can't understand how they just need to have access to one of my pages to create ads for it.
As far as I understand you need to be a business manager administrator to claim a page as agency with the ADVERTISER role (cf. https://developers.facebook.com/docs/marketing-api/businessmanager/assets#pages) but
If the user who makes an AGENCY claim call does not have the proper permissions on the Page, the response will be PENDING. The Admin for that Page may login and grant the access, deny it, or even report the claim as a spam.
So how come Mailchimp doesn't require this validation?
So there is another way which doesn't involve business manager directly: adding a system user as advertiser.
Assigning permissions to a page from the API requires special permissions. Please reach out to your Facebook rep if you require this feature.
My team is still in the process to “reach a facebook rep” (it's been two weeks already…) but once this will be done a simple POST to <PAGE_ID>/roles with
admin_id: system user IDrole: Advertisershould allow to create ads for pages using the system user access token.
Turns out facebook desn't want you to use the features they advertise, so there's in fact no solution.