Okay I fixed it with this:
var splitPoint = content.IndexOf(":", 1);
var endpoint = content.IndexOf(",", 1);
var splitClient_id = content.Substring(1);
splitClient_id = content.Substring(splitPoint + 2, endpoint - splitPoint - 3);
Debug.Log(splitClient_id);
HttpRequestMessage request = new HttpRequestMessage(HttpMethod.Post, $"https://id.twitch.tv/oauth2/revoke?client_id={splitClient_id}&token={OAuth_Token}");
HttpResponseMessage response = await httpclient.SendAsync(request);
Debug.Log(request);
Thank you for your help 