public static ArcGISPortalManager Current {get;}

var active_portal = ArcGISPortalManager.Current.GetActivePortal();
string uri = active_portal.PortalUri.ToString();

var portals = ArcGISPortalManager.Current.GetPortals();
//Make a list of all the Uris
var portalUris = portals.Select(p => p.PortalUri.ToString()).ToList();

var portalUri = new Uri("http://myportal.esri.com/portal/", UriKind.Absolute);
ArcGISPortalManager.Current.AddPortal(portalUri);

//Find the portal to sign in with using its Uri...
var portal = ArcGISPortalManager.Current.GetPortal(new Uri(uri, UriKind.Absolute));
if (!portal.IsSignedOn())
{
  //Calling "SignIn" will trigger the OAuth popup if your credentials are
  //not cached (eg from a previous sign in in the session)
  if (portal.SignIn().success)
  {
    //Set this portal as my active portal
    ArcGISPortalManager.Current.SetActivePortal(portal);
  }
}