Appears that only Bitbucket Online is currently supported with CommitStream. Webhooks is not a feature of Bitbucket Server (on-premise version of Bitbucket).
If it is supported it would appear to be undocumented at this time.
by: Ray W. | over a year ago | Integration
Comments
It looks like it has support for webhooks according to this:
https://confluence.atlassian.com/bitbucketserver/post-service-webhook-for-bitbucket-server-776640367.html
There is a free integration for setting up post receive hooks:
https://marketplace.atlassian.com/plugins/de.aeffle.stash.plugin.stash-http-get-post-receive-hook/server/overview