Start Blazor circuit with custom URL (#34404)

Author: guardrex

aspnetcore/blazor/fundamentals/signalr.md

@@ -1452,6 +1452,61 @@ Use a <xref:Microsoft.AspNetCore.Components.Server.Circuits.CircuitHandler> to c
 
 :::moniker-end
 
+## Start the SignalR circuit at a different URL
+
+Prevent automatically starting the app by adding `autostart="false"` to the Blazor `<script>` tag ([location of the Blazor start script](xref:blazor/project-structure#location-of-the-blazor-script)). Manually establish the circuit URL using `Blazor.start`. The following examples use the path `/signalr`.
+
+:::moniker range=">= aspnetcore-8.0"
+
+Blazor Web Apps:
+
+```diff
+- <script src="_framework/blazor.web.js"></script>
++ <script src="_framework/blazor.web.js" autostart="false"></script>
++ <script>
++   Blazor.start({
++     circuit: {
++       configureSignalR: builder => builder.withUrl("/signalr")
++     },
++   });
++ </script>
+```
+
+Blazor Server:
+
+:::moniker-end
+
+```diff
+- <script src="_framework/blazor.server.js"></script>
++ <script src="_framework/blazor.server.js" autostart="false"></script>
++ <script>
++   Blazor.start({
++     configureSignalR: builder => builder.withUrl("/signalr")
++   });
++ </script>
+```
+
+Add the following <xref:Microsoft.AspNetCore.Builder.ComponentEndpointRouteBuilderExtensions.MapBlazorHub%2A> call with the hub path to the middleware processing pipeline in the server app's `Program` file.
+
+:::moniker range=">= aspnetcore-8.0"
+
+Blazor Web Apps:
+
+```csharp
+app.MapBlazorHub("/signalr");
+```
+
+Blazor Server:
+
+:::moniker-end
+
+Leave the existing call to <xref:Microsoft.AspNetCore.Builder.ComponentEndpointRouteBuilderExtensions.MapBlazorHub%2A> in the file and add a new call to <xref:Microsoft.AspNetCore.Builder.ComponentEndpointRouteBuilderExtensions.MapBlazorHub%2A> with the path:
+
+```diff
+app.MapBlazorHub();
++ app.MapBlazorHub("/signalr");
+```
+
 ## `IHttpContextAccessor`/`HttpContext`
 
 [!INCLUDE[](~/blazor/security/includes/httpcontext.md)]