When you send a serverside subscription to a webhook, is the node that triggered the subscription sent in the request?

Yes, you define the server side subscription in form of a query like client side subscription. The result of the query is the payload! Like https://github.com/prismagraphql/prisma/blob/master/examples/server-side-subscriptions/prisma/prisma.yml#L6 and https://github.com/prismagraphql/prisma/blob/master/examples/server-side-subscriptions/prisma/subscription.graphql 👍