In the HTTP POST header you are sending, are you sending to start the stream, ContentType = "application/json;charset=utf-8"
The guy here has a GitHub repo. It may help.
This is old, but maybe still works.
string url = "Enter HTTP path of THETA here";
var request = HttpWebRequest.Create (url);
HttpWebResponse response = null;
request.Method = "POST";
request.Timeout = (int) (30 * 10000f); // to ensure no timeout
request.ContentType = "application/json; charset = utf-8";
byte [] postBytes = Encoding.Default.GetBytes ( "Put the JSON data here");
request.ContentLength = postBytes.Length;
This example works with V (I have not tested)
Another example with the V. See the updated zip file
THETA S WiFi Streaming with Unity - #20 by ZHU
BTW, if you have a moment, can you share your use case? I’m always curious as to the use of the 360 video streaming.