Finally I solve myself. My mistake was that I was trying to post json(either encoded or what) in URL for request, while server may not have any method to support such request. Also I tried with ASIHTTPRequest, don't know why ASIFormRequest was not working. Now I am getting response.
So I changed above code as:
NSURL *url = [NSURL URLWithString:[NSString stringWithFormat:@"http://api.esha.com/analysis?apikey=%@",ApiKey]];
ASIHTTPRequest *request = [ASIHTTPRequest requestWithURL:url];
[request addRequestHeader:@"Content-Type" value:@"application/json"];
[request setPostBody:(NSMutableData *)[jsonString dataUsingEncoding:NSUTF8StringEncoding]];
[request setDelegate:self];
[request startSynchronous];