Begin communicating with the remote server.
After calling this method, the UnityWebRequest will perform DNS resolution (if necessary), transmit an HTTP request to the remote server at the target URL and process the server’s response.
This method can only be called once on any given UnityWebRequest object. Once this method is called, you cannot change any of the UnityWebRequest’s properties.
This method returns a WebRequestAsyncOperation object. Yielding the WebRequestAsyncOperation inside a coroutine will cause the coroutine to pause until the UnityWebRequest encounters a system error or finishes communicating.