Hi Nagendra,
By default, external Api only does a GET call to the external endpoint. In order to do a POST call, there has to be some json in Post Body Template section under Inbound Mapping tab in External Api activity.
1. Either you can add a query (or similar) activity before externalApi activity and define some field mapping in the Inbound columns section.
2. Or you can add some random valid json in the Customize Parameter section.
This will then get added to the postbody template section and will make a POST call to your endpoint.
-Pushkar