Send data using Message scenario trigger

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Hi Gurus,

I am trying to send a custom field status to external system(REST API/ SOAP enabled) when an activity is started. Aware this could be done using message scenario, but can anyone share the steps on how to call the external system in channels and the payload which has to be given in Message Scenario-> patterns->Body and Subject. It would be great help if any one shares any payload(SOAP or REST) for our reference.

Thanks,

Madhu