When you call a service from PostMan you give all the detail directly through UI which is quite easy but in PEGA when DP is sourced from Connector you need to check how request parameter is given to service eg Request/Response Data transforms etc.
You may also delete the existing DP instance [ Load Mgmt - Clear Data page] and re execute the scenario and trace it to check the detail of REST request and response parameters.
You can directly run and trace DP to see request and response parameter details.
Hope this will help. If yes then please mark as helpful/correct.