Create an instance of the AspNetSynchronizationContext, which is internal and is not easily constructed.

CreateAspNetSynchronizationContext.cs

static SynchronizationContext CreateAspNetSynchronizationContext()
{
    const string taskFriendlyConfigName = "aspnet:UseTaskFriendlySynchronizationContext";
    var originalConfigSetting = ConfigurationManager.AppSettings[taskFriendlyConfigName];
    ConfigurationManager.AppSettings[taskFriendlyConfigName] = "true";
    try
    {
        TextWriter writer = new StringWriter();
        HttpRequest rx = new HttpRequest("", "http://example.com", "");
        HttpResponse tx = new HttpResponse(writer);
        HttpContext ctx = new HttpContext(rx, tx);
        ctx.AllowAsyncDuringSyncStages = true;

        return (SynchronizationContext)ctx.GetType().GetProperty("SyncContext", BindingFlags.Instance | BindingFlags.NonPublic).GetGetMethod(true).Invoke(ctx, null);
    }
    finally
    {
        ConfigurationManager.AppSettings[taskFriendlyConfigName] = originalConfigSetting;
    }
}