The basic Instagram Account option is located in the Profile section in the People menu. It’s listed alongside the Sync contacts and Blocked people options. Once a user syncs their Instagram account with Messenger, their Instagram username and account will be visible to people who they see on Messenger.
This new feature was discovered by Jane Manchun Wong who has dug into many apps, including some from Facebook, in the past to discover unreleased features.
This option is reportedly available to a small number of people already which suggests that Facebook is only testing it at this point in time. If it’s convinced about the feature’s usability in real life then we can expect to see this feature being widely rolled out in the future. It can’t be said for sure right now when that’s going to happen, though.