I know there are Sub Processes, but what I'd like to do is have a separate Forms Business Process that can be run ad-hoc, or be initiated by the completion of another Form.

I am confident there isn't a way to do this in the UI, but what about through a Webhook? Can Webhook be used to hit the Forms web service and tell it to start a Business Process with certain parameters?

Use case:

For an HR workflow, I have an Employment Application that leads in to an Onboarding workflow. But if the employee doesn't get hired through the Employment Application process I'd still like to be able to assign them the Onboarding process. Rather than duplicate all of the work in to two different processes, I would like to just be able to kick off Onboarding ad-hoc or be triggered by the end of the Application process.