Hi Arkesh,
All you need to do in addition to the above steps is configure the property PostData in HttpPost action block in addition to the properties configured above. This property is of type string and should be the XML document which you want to send across to PI.
The configuration will be done in the same way as you have configured URL, User id etc.
When you run the transaction, your message will be posted to PI.
Regards,
Anushree