CSAT through Facebook messenger integration with Freshdesk

Hi,

We are developing app to gather CSAT through facebook messenger and store the CSAT rating into the freshdesk.

This CSAT would be sent to customer when ticket is closed.

I need PSID to send the CSAT through the facebook messenger window.

I don’t find any API to get PSID or any other ways to get PSID or any other different approach to send CSAT to specific customer’s facebook messenger