Merge two project blazor server

Question

I have two project (blazor server), and i need mixed/merged him. I find this solution and i try adapted for use same logique. But in Program when i try using mapWhen i cant.

app.MapWhen(ctx => ctx.Request.Path.StartsWithSegments("/projet1"), app1 =>
{
    app1.UseBlazorFrameworkFiles("/projet1"); //in Server this not the same usage, not supported
    app1.UseRouting();
    app1.UseEndpoints(endpoints =>
    {
        endpoints.MapFallbackToFile("/projet1/{*path:nonfile}", "/projet1/_Host");
    });
});

starting point this question.

Also, i donlowad projet, for debug, but i cant, i see that someone also in 2022 made the same comment

Maybe my english can be better, but i try explain.