To add MCS as a partner,  you will need to go to your Meta Business Manager account. Once there, click the Settings gear icon down on the bottom-left, then click on “Partners,” and then, to the right, click “Add.”

You’ll get the following pop-up (below.)  Select the first option:

Plug in the business ID for MCS in the dialog box: 275276109996099

When you move forward, you’ll see this…

On the left-hand menu, you’ll pick the assets to share: Pages, Pixels, Instagram accounts. Then, scroll to the bottom and click Full Control.