Startup.cs

//Add using statements:
//using Xamarin.Forms;
//using Ooui.AspNetCore;

//Add these in Configure:
//app.UseOoui();
//Forms.Init();

public void Configure(IApplicationBuilder app, IHostingEnvironment env)
{
    if (env.IsDevelopment())
    {
        app.UseDeveloperExceptionPage();
    }
    else
    {
        app.UseExceptionHandler("/Home/Error");
    }

    app.UseStaticFiles();

    app.UseOoui();

    Xamarin.Forms.Forms.Init();

    app.UseMvc(routes =>
    {
        routes.MapRoute(
            name: "default",
            template: "{controller=Home}/{action=Index}/{id?}");
    });
}