To create this trigger, first navigate to the "Triggers" tab and then scroll down to find the "Start a flow when Facebook refers a contact" option:
Next, choose the channel you want this trigger to operate on and the flow you want to start:
Note that the referrer ID is optional. It can be embedded in a link to be customized with the flow you use. For more information on referrals see here.
Once you've created this trigger, the flow will be started when a contact initiates a conversation with Facebook Messenger on your page.
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