We want to set the "Converted from Lead" value in Deals using a Workflow or via a Deluge script. How?

We want to set the "Converted from Lead" value in Deals using a Workflow or via a Deluge script. How?

For use in Zoho Analytics, we need the field "Converted from Lead" filled in our deal records.

This field is empty everywhere, because we do not create deals directly when converting a lead to a contact.

We want to do that using the API or a workflow or a Deluge script.
How can we update this field in existing deals, or while creating a deal within an existing contact?