While your ads will post on Instagram by default, even if your IG page isn't correctly connected to your Facebook page, it's best you do take some time to connect the two!
The main benefit of doing so that users who click on your Instagram ads get directed to your real profile. Users who click go to a dummy profile if you haven't linked your Instagram account with a Facebook page.
There are two primary methods for linking an Instagram Business Profile to a Facebook Page.
The first method is to link the Instagram Business Profile to the Facebook Page through your Facebook Page Settings.
The second method is to link the Facebook Page while logged into the Instagram account on the Instagram mobile app.
Both methods require you to be a Facebook Page Admin.
Method 1: Link the Instagram Business Profile to the Facebook Page via Facebook Page Settings
You can add an Instagram account to a Facebook Page for which you are an admin or editor. Adding an Instagram account to your Page allows you to create Instagram ads in Ads Manager without connecting your Instagram account to a Business Manager.
Add an Instagram account to your Facebook Business Page
To add an existing Instagram account to your Facebook Page:
Go to your Facebook Page.
Click Settings in the bottom-left of your Page.
Scroll down on the left-hand side and select Instagram.
To add an existing Instagram account to your Page, click Add an Account.
Enter your Instagram account's Username and Password, and click Confirm.
You should now have an Instagram account connected to your Facebook Page.
Whenever you use your Facebook Page to create ads that will run on Facebook, you can use your new Instagram account to show those same ads on Instagram.
Method 2: Link Facebook Page While Logged into the Instagram Account on the Instagram Mobile App
To link your Instagram account to your brand's Facebook Page, you first need to be an admin of that Page on Facebook.
You can view a video guide here:
A video on how to Link an Instagram Account to a Facebook Page from the Instagram App.
To link your Instagram account to a Facebook Page:
Android
Go to your profile and tap in the top right. Scroll down and tap Linked Accounts, then select Facebook.
If you haven't already, enter your Facebook login information.
By default, your Instagram account will link to your personal Facebook Timeline. To link to a Page instead, tap Facebook again and choose a Page that you manage below Share to.
iPhone or Windows Phone
Go to your profile and tap in the top right. Scroll down and tap Linked Accounts, then select Facebook.
If you haven't already, enter your Facebook login information.
By default, your Instagram account will link to your personal Facebook Timeline. To link to a Page instead, tap Facebook again, then tap Share to, and choose a Page that you manage.