Custom URL to launch Facebook Messenger on iOS

Just use https://m.me/USER_ID.

This will deep link the user to the native Messenger app if available and start a conversation with the user identified by its USER_ID.

If the native app is not available on the device, it will open the messenger web app in a new browser.

The URL scheme is fb-messenger://user-thread/{user-id}. I just build my app with fb-messenger URL scheme as Viewer and trapped the URL on the UIApplication delegate. The user-id parameter is long integer one, as in https://developers.facebook.com/docs/graph-api/reference/user/