Zendesk verification

I need to display the user's name in the Zendesk chat (for the agents), and I'm using the Zendesk code:

window.zESettings = {
 webWidget: {
   authenticate: {
     chat: {
       jwtFn: function(callback) { 
         fetch('JWT_TOKEN_ENDPOINT').then(function(res) {
            res.text().then(function(jwt) {
             callback(jwt);
            });
          });
        }
      } 
    }
  }
};

and the jwt token is as follows:

var payload = {
  name: '#{customerName}',
  email: '#{customerEmail}',
  iat: #{timestamp},
  external_id: '#{externalId}'
};
var token = jwt.sign(payload, '#{yourSecret}');

The code does not function, and there are no problems, but the user name is not shown. The Zendesk manual is devoid of useful information; has anyone incorporated Zendesk into their app? What am I doing incorrectly? Any assistance and suggestions are much welcomed.