Please keep in mind that you can not declare an Instagram account you don't own. If you are promoting on the behalf of another person, the owner should initially declare their Instagram account through their own Facebook Service Manager.
How To Link Your Instagram And Facebook
If you own the Instagram account: Just go to your Facebook Business Supervisor and click settings. Under individuals and Assets tab, click Instagram Accounts on the left then click on Claim Instagram Account button on the right.
Type in your Instagram username and password.
You can also decide to connect it to your advertisement account if you have one.
Furthermore you can also appoint partners to handle your Instagram account after you've link Facebook Service Manager to your Instagram account.
Back in the advertisements supervisor page, when you've successfully added the account you'll have the ability to select your Instagram account at the advertisement development page.
Doing this will enable people to click to follow you on Instagram instead of sending them to your Facebook page.
Remember that you can not directly buy fans on Instagram instead of Facebook likes project How To Link Your Instagram And Facebook.