Dear all, Thanks for your response now it is resolved.
I'm unable to understand why do you want to pass postman-token in this request.
For bearer token try this --
post.setHeader("Authorization", "Bearer " + token);
httpClient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", "Your Oauth token");
For passing json, you have already written the code to set the body/json in HttpEntity -- post.setEntity(requestEntity);