This actually sounds like a bug of the extension that should use the standard field since one exist. Would be great if you could create an issue on Loading....
I don’t really know this extension but changing the field used is probably easy. The source can be found on GitHub - xwiki-contrib/application-piwik: Extension to add Piwik tracking to an XWiki instance and the interesting part here seems to be application-piwik/src/main/resources/XWiki/PiwikCode.xml at master · xwiki-contrib/application-piwik · GitHub. On your side it means editing XWiki.PiwikCode wiki page with object editor and open the XWiki.UIExtensionClass object.
Pull request is very welcome if you can make something more standard from Matomo point of view work 