INSTAGRAM ADVERTISING: connecting IG account to Facebook Business Manager
Despite Facebook's claim that one does not need Business Manager in order to advertise on Instagram, you should never do so. No professional or agency should ever use Facebook's "one-click" advertising solutions represented by the infamous "boost post" button. These are black holes with limited targeting and adjustment options and you should always take the longer, more complicated, but significantly more rewarding path of the Power Editor, 3rd party tool or at least Facebook's Ads Manager.
So, how to connect an Instagram account to your Facebook ads account? Once you get on the right track, it's very easy. The only trick dwells in the fact that this process happens in the Facebook Business Manager settings, not in the Ads Manager.
Select Instagram Accounts from the menu
The button you are looking for is in the top right corner.
Input IG account handle (@johnsmith)
Successful operation results in the IG account becoming part of the connect Instagram accounts overview. To remove the account, simply click on "Remove" button after selecting the right account to be deleted from the list below.
Subsequently, you will have to assign the added Instagram account to a selected Ads account (Facebook expects that mainly agencies operate multiple advertising accounts, in fact, pushes us to separate ad accounts of each brand).