If you can't find your Facebook Ad Account in the dropdown menu when connecting accounts, that means you are using your personal Ad Account to run ads or you are using the Business owned Ad Account but your personal Facebook account has not been assigned as an Admin role into the Ad Account under your Business Manager.
(1) If you are using your personal Ad Account to run ads, in order to add other apps or agencies as your partners, you need to add your personal Ad Account to your Business Manager first.
Please go to your Facebook Business Manager and click the button "Add an Ad Account". Type your personal Ad Account ID and finish the transition. After the action, you can manage all of your Ad Accounts from your Business Manager and leverage more powerful features.
(2) If you have already had Ad Accounts in your Business Manager but haven't seen it from the dropdown menu, you need to go to your Ad Account setting from Business Manager and then assign your account as an Admin role.
Please click the button "Add People" in your Ad Account setting, and then assign your account with an Admin role.