I have a Facebook Messenger group which is initially controlled by a bot, which we wish to configure for agent escalation.  Now I understand that I just need to configure this facebook group as a channel with the bot as the primary controller and enable handover protocol

As seen in this link https://developers.ringcentral.com/engage/digital/guide/interactions/bots-api   
I just need to call 

curl -X POST "https://domain-test.api.engagement.dimelo.com/1.0/bots/handover?access_token=<access_token>&identity_foreign_id=<foreign_id>&from=bot&to=agent&type=messenger"

When handing over from bot to agent, what is the 'identity_foreign_id' and where do I find it ?