Webpush config in notification payload

[Jyra Nevmerzhitsky]

According to the documentation (https://firebase.google.com/docs/reference/fcm/rest/v1/projects.messages), in a Firebase push notification payload, I can have a webpush object with some settings.  

I have set up Firebase as described here - https://firebase.google.com/docs/cloud-messaging/js/receive. But according to the documentation (https://firebase.google.com/docs/reference/js/messaging_.notificationpayload), the onMessage function will receive only the base template notification payload. How can I receive the webpush config?
P.S. I'm sending webpush in the payload from the server side.