Here, 1 NuGet package is used for showing watermark-
https://www.nuget.org/packages/Gu.Wpf.Adorners/
if you are getting error using that, please run this command in NuGet command prompt-
Get-Package
Even if not working, then please try this-
Install-Package Gu.Wpf.Adorners
Uninstall-Package Gu.Wpf.Adorners
Install-Package Gu.Wpf.Adorners
Thanks