You can do that setting here:
Step 1. Click on "Add Inbox" button from Settings > Inboxes page.
Step 2. Click on the "Messenger" icon.
Step 3. Click on the Facebook login button. It will open a new window for you to log in.
Step 4. Authenticate with Facebook, select an Instagram account and select the page you want to connect. Enable all permissions shown in the list. Otherwise, the app might not work.
If you have previously connected your OMNI to your FB Messenger page, you can override it and use this inbox as combined Facebook and Instagram Messenger inbox. You can also deselect the pages you do not want to be part of OMNI. In this example we combine it. We recommend to combine your FB pages with IG account.
Step 5. "Add agents" to your FB/IG Messenger inbox.
Step 6. Hooray! You have successfully created a Messenger/IG inbox. Whenever a customer sends a message to your Instagram account or Facebook page, you will see it in the dashboard.
Step 7. If you want to update the agents who have access to the inbox, you can go to Settings > Inboxes.