If you own a business and are having a person or agency manage your Facebook ads account, here is how to grant them access to your account and your Facebook Page.
In this method, you’ll need to ask them for their Facebook Business ID:
Go the Ad Accounts tab in Business Manager Settings: https://business.facebook.com/settings/ad-accounts/
Select the Ad Account
Click Assign Partners
Assign Partner by: Business ID
Enter their Business ID
Select the access of “Manage campaigns” (or “Manage Ad Account” if you want to give them Admin access)