We had this problem for a while, just this morning I was able to set Content-length header for POST method, but now it doesn't work anymore. Why is it? I haven't done anything, I heard that this is restricted but I could use it just fine yesterday:
request.Headers["Content-Length"] = byteArray.Length.ToString();
Copyright Notice：Content Author:「Nghia Nguyen」，Reproduced under the CC 4.0 BY-SA copyright license with a link to the original source and this disclaimer.
Link to original article：https://stackoverflow.com/questions/7332339/cant-set-content-length-header-for-post-method