The data-user-id is your end user's ID and could be your user's email id or a username. It is the ID that will be associated with the authorization and the data that is communicated between applications once authorized.
The data-user-id is requested in Step 1 and Step 2 of Deploy.
Requested in the <head> script tag
Requested in the message sending code base
Need help? Reach out to us at firstname.lastname@example.org and we would be happy to assist!