public PopupContent( 
   string htmlContent,
   string title
)

public void ShowCustomPopup()
{
  //Get the active map view.
  var mapView = MapView.Active;
  if (mapView == null)
    return;

  //Create custom popup content
  var popups = new List<PopupContent>
        {
            new PopupContent("<b>This text is bold.</b>", "Custom tooltip from HTML string"),
            new PopupContent(new Uri("http://www.esri.com/"), "Custom tooltip from Uri")
        };
  mapView.ShowCustomPopup(popups);
}

public void ShowCustomPopupWithWindowDef()
{
  if (MapView.Active == null) return;

  //Create custom popup content
  var popups = new List<PopupContent>
        {
            new PopupContent("<b>This text is bold.</b>", "Custom tooltip from HTML string"),
            new PopupContent(new Uri("http://www.esri.com/"), "Custom tooltip from Uri")
        };
  // Sample code: https://github.com/ArcGIS/arcgis-pro-sdk-community-samples/blob/master/Framework/DynamicMenu/DynamicFeatureSelectionMenu.cs
  var topLeftCornerPoint = new System.Windows.Point(200, 200);
  var popupDef = new PopupDefinition()
  {
    Append = true,      // if true new record is appended to existing (if any)
    Dockable = true,    // if true popup is dockable - if false Append is not applicable
    Position = topLeftCornerPoint,  // Position of top left corner of the popup (in pixels)
    Size = new System.Windows.Size(200, 400)    // size of the popup (in pixels)
  };
  MapView.Active.ShowCustomPopup(popups, null, true, popupDef);
}