How to authenticate user credentials on Azure Active Directory without invoking the microsoft login UI?

I have username and password,
I'd like to authenticate these with the Azure Active Directory in my cloud, and I want it to be done with Microsoft Graph API if possible.
And most important, absolutely no user interface. Assume its all running via command line, or in a script.

So far I got the UI way (using C# but ultimately it will be C++):

Uri authUri = new Uri(string.Format("https://login.microsoftonline.com/{0}/oauth2/v2.0/authorize", TenantId));
try
{
   string data = string.Format("client_id={0}&response_type=code&response_mode=query&scope=user.read&redirect_uri={1}", ClientId, WebUtility.UrlEncode(redirectUrl));
   BrowsePage b = new BrowsePage();
   b.navigate(authUri, data);
   b.Show();

   b.Closed += new EventHandler((object sender, System.EventArgs e) =>
   {
      is_authenticated = true;
   });
}
catch (System.Net.WebException ex)
{
    System.Console.WriteLine("Authorize request: " + ex.Message);
}