a

//https://ryansouthgate.com/asp-net-core-cors-wildcard-subdomains/
public virtual void ConfigureServices(IServiceCollection services)
{
    //...other ASP.NET Setup

    services.AddCors(options =>
            {
                options.AddDefaultPolicy(
                    builder => builder
                    .WithOrigins
                    (
                        "https://*.ryansouthgate.com"
                    )
                    // This method call is important!
                    .SetIsOriginAllowedToAllowWildcardSubdomains()
                    .AllowAnyMethod()
                    .AllowAnyHeader());
            });
}