When setting up an account with GMX.com or web.de using the Automatic Setup, both the CalDAV and CardDAV will be added automatically.
If you want to add it manually, the urls are:
GMX.com
CalDAV: https://calendar.gmx.com/begenda/dav/users/
CardDAV: https://carddav.gmx.com/.well-known/carddav/
web.de
CalDAV: https://kalender.web.de/begenda/dav/users/
CardDAV: https://carddav.web.de/.well-known/carddav/