Hi! Based on Dan's extensions and heavy cut'n paste etc... I managed to build an extension which displays a popup window with an external link. Problem is the extension first creates a link which then has to be clicked to show the external page. So there is one link too much in the chain. ;-)
Here is the code: https://github.com/chris58/isis-wicket-popupbox.git If anybody has an idea for how to let the extension pop up the window without the use of an ExternalLink please let me know. :-) Regards Christian
