Unfortunately, this is not a use case we support in a first-class manner - these projects aren’t returned from the Projects and Workspaces endpoints, and even querying on tasks in the project that are only in one’s “My Tasks” come up as having no projects associated with them.
If copying the ID from the URL will not work (say you’re trying to automate this for multiple users), I would suggest instead creating a new project, having a user add their tasks to that project, and then use that project for webhook tracking.