IPAddress server(192,168,1,120); // server IP address
char PostData[] = "{\"name\": \"Fred\", \"age\": 31}"; // your JSON payload

.....

if (client.connect(server, 80)) {
  client.println("POST /doit HTTP/1.1");
  client.println("Host: 192.168.1.120"); // or generate from your server variable to not hardwire
  client.println("User-Agent: Arduino/unowifi");
  client.println("Connection: close");
  client.print("Content-Length: ");
  client.println(strlen(PostData));// number of bytes in the payload
  client.println();// important need an empty line here
  client.println(PostData);// the payload
}