Fetch existing private message channels on re-joining

osu.Game/Online/API/Requests/CreateChannelRequest.cs

@@ -0,0 +1,34 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using System.Linq;
+using System.Net.Http;
+using osu.Framework.IO.Network;
+using osu.Game.Online.API.Requests.Responses;
+using osu.Game.Online.Chat;
+
+namespace osu.Game.Online.API.Requests
+{
+    public class CreateChannelRequest : APIRequest<APIChatChannel>
+    {
+        private readonly Channel channel;
+
+        public CreateChannelRequest(Channel channel)
+        {
+            this.channel = channel;
+        }
+
+        protected override WebRequest CreateWebRequest()
+        {
+            var req = base.CreateWebRequest();
+            req.Method = HttpMethod.Post;
+
+            req.AddParameter("type", $"{ChannelType.PM}");
+            req.AddParameter("target_id", $"{channel.Users.First().Id}");
+
+            return req;
+        }
+
+        protected override string Target => @"chat/channels";
+    }
+}