you will have to make a http POST request to the url http://x.x.x.x/rest/workflows/<UUID of the workflow>/jobs.
 
Add http header Content-Type as "application/xml"
 
The http body should be xml like this
<?xml version="1.0" encoding="UTF-8"?>
<workflowInput>
   <userInputValues>
      <key>ClusterName</key>
      <value>abc</value>
      <key>VserverName</key>
      <value>vs1</value>
      <key>VolumeName</key>
      <value>DemoVol023</value>
   </userInputValues>
</workflowInput>