This is simple example of how to create a plugin for Stash that will trigger a URL webhook in Jenkins.
When running Stash you can enable the webhook for a specific repository via the Repository Hook configuration.
Where $URL will be replaced with the HTTP url of the repository, and $BRANCHES contains a comma separated list of branches.
To build this plugin use the standard Atlassian Plugin SDK and/or run the following (must be Maven 2.1.0):
mvn install
And upload target/stash-webhook-plugin-2.0.0-SNAPSHOT.jar to Stash.