To add an advertising account to your Business Manager: Open your Business Manager Settings. Under the People and Assets tab, click on “Ad Accounts”. On the right side of the page, select “Add New Ad Accounts”. Choose one of the 3 options: “Claim Ad Account”, “Request Access to an Ad Account” or “Create a New Ad Account”.
https://adespresso.com/guides/facebook-ads-beginner/setting-up-your-facebook-ads-account/

