HttpClient httpclient = new HttpClient();
var postParameters = new List<KeyValuePair<string, string>>();
postParameters.Add(new KeyValuePair<string,string>("INSTITUTION_ID", "isntitution_id");
postParameters.Add(new KeyValuePair<string,string>("AGGREGATOR_ID", "aggregator_id");
postParameters.Add(new KeyValuePair<string,string>("MEMBER_ID", "member_id");
postParameters.Add(new KeyValuePair<string,string>("PASSWORD", "password");
postParameters.Add(new KeyValuePair<string,string>("inputJson_", "json string");
var content = new FormUrlEncodedContent(postParameters);
var response = await httpclient .PostAsync(url, content);
var responseString = await response.Content.ReadAsStringAsync();
Don't just Copy paste the code directly .first debug and identify if you are getting any errors in some lines mentioned above then find out the problem and if you aren't able to solve post your comments and if you find any issues and solved it your own then comment how you resolved as well