how to set accept header application/vnd.api+json in HttpWebRequest

i am trying to call webshipper api . api needs these header

Accept: application/vnd.api+json
Accept-Encoding: gzip, deflate
Accept-Language: en-GB,en-US;q=0.9,en;q=0.8

here is my code

HttpWebRequest request = (HttpWebRequest)WebRequest.Create("https://bascrm.api.webshipper.io/v2/rate_quotes");
    request.Method = "POST";
    request.KeepAlive = true;
    request.Accept = "application/vnd.api+json";
    request.ContentType ="application/vnd.api+json";
    request.Headers.Add("Authorization", "Bearer " + token + "");

    WebHeaderCollection myWebHeaderCollection = request.Headers;
    myWebHeaderCollection.Add("Accept-Language", "en-GB,en-US;q=0.9,en;q=0.8");
    myWebHeaderCollection.Add("TransferEncoding", "gzip,deflate");

i getting error 400 bed request

when i change request.Accept = "application/vnd.api+json"; to

request.Accept = "application/json";

it will give error 406 not acceptable