If you're looking to figure out how to add agencies to your Facebook business page, you can do this in a few different ways.
If you're an Agency on Facebook, you would follow these steps. If your a business owner looking to accept a request sent and don't know where to go, click here.
Step 1: Go to https://business.facebook.com/
Step 2: Select your business and then click the blue button called Business Settings.
Step 3: Click Pages on the left, and then in the area that opens up, click the blue button called Add, then Request Access to a Page
Step 4: In the popup all you have to do is type the Facebook page name or paste the URL and click Request Access
Step 5: Tell the business owner to go to their Facebook settings, then page roles.