https://learn.microsoft.com/en-us/uwp/api/windows.graphics.printing.printtasksourcerequestedhandler?view=winrt-28000
PrintTaskSourceRequestedHandler Delegate (Windows.Graphics.Printing) - Windows apps | Microsoft...
Occurs when a print task requests the document to print. The supplied print document must implement the IPrintDocumentSource interface.
windows graphicsdelegateprintingappsmicrosoft