Webhooks of a model are created by mapping of fields to values.
POST /restapi/1.0/webhooks?vals={values_for_the_object's_fields}
Request:
POST /restapi/1.0/webhooks?vals={'name':'Delivery Order Updation','model':'stock.picking','kind':'on_write','address':'https://requestbin.net/152eq5l1','format':'json','language':'en_US','condition':[('state', '=', 'done'), ('picking_type_id.code', '=', 'outgoing')]} HTTP/1.1
Host: <your Odoo server url>