How do we give a default payload to start the workflow.
I am not sure what you mean, However - I am assuming you meaning content such as /content/Geometrixx. That is - how do we start a workflow based on something happening to /content/Geometrixx.
The answer is to use Workflow Launchers.
You can read about them in this article:
https://helpx.adobe.com/experience-manager/using/Workflows.html
Also - there is an Ask the AEM Community Experts on Workflows at the start of this article that explains a lot about workflows.