When you start creating a Facebook Messenger Zap, you will be asked to connect your Facebook Messenger account.
You’ll now be asked to log into your Facebook Messenger account (unless you are already logged in).
You will be asked if you want to continue as a particular user.
You will be asked what Facebook Pages you want to connect.
Finally, you will be asked to give Zapier permission to access your account. Click "Done” to continue.
It would then tell you that you've linked the Zapier Messenger app to Facebook.
If all steps were successful your Facebook Messenger account will now be successfully connected.