ArcGIS Pro 2.7 API Reference Guide
HtmlURI Property
Example 

ArcGIS.Desktop.Mapping Namespace > PopupContent Class : HtmlURI Property
Gets or sets the path to the html content to display in the pop-up.
Syntax
public Uri HtmlURI {get; set;}
Public Property HtmlURI As Uri
Example
Show a custom pop-up.
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>();
  popups.Add(new PopupContent("<b>This text is bold.</b>", "Custom tooltip from HTML string"));
  popups.Add(new PopupContent(new Uri("http://www.esri.com/"), "Custom tooltip from Uri"));
  
  mapView.ShowCustomPopup(popups);
}
Requirements

Target Platforms: Windows 10, Windows 8.1

See Also

Reference

PopupContent Class
PopupContent Members