Model for the Referral Callback.
This callback will occur when an m.me link is used with a referral param and
only in a case this user already has a thread with this bot (for new threads
see
Postback Event). See also the full guide on
m.me links.
To start receiving these events you need to subscribe to messaging_referral
in the webhook settings for your app.
An m.me link with an an added parameter looks like this:
http://m.me/mybot?ref=myparam. The value of the ref parameter will be passed
to the server via webhook.