yep

Shogi.UI/Pages/Home/GameBoard/SpectatorGameBoard.razor

@@ -1,7 +1,27 @@
 @using Contracts.Types;
+@using System.Net;
+@inject IShogiApi ShogiApi;
 
-<GameBoardPresentation IsSpectating="true" Perspective="WhichPlayer.Player1" Session="Session" />
+<GameBoardPresentation IsSpectating="true"
+                       Perspective="WhichPlayer.Player1"
+                       Session="Session"
+                       OnClickJoinGame="OnClickJoinGame" />
 
 @code {
     [Parameter] public Session Session { get; set; }
+
+    protected override void OnParametersSet()
+    {
+        base.OnParametersSet();
+        if (Session == null)
+        {
+            throw new ArgumentException($"{nameof(Session)} cannot be null.", nameof(Session));
+        }
+    }
+
+    async Task OnClickJoinGame()
+    {
+        var response = await ShogiApi.PatchJoinGame(Session.SessionName);
+        response.EnsureSuccessStatusCode();
+    }
 }