What is the channel code for Mediathek for Roku?
There's no channnel for ZDF Mediathek. ZDF
(Zweites Deutsches Fernsehen = Second German Television) is one of the two German public-service TV networks which are funded by a television licence. This doesn't allow them to offer a channel for the RSP.
Furthermore many people over here think it's a real shame that the content of ZDF Mediathek and other German public-service TV and radio stations is freely available worldwide whereas major US broadcast networks geo-lock their stations.
If you want a channel for German TV, you could ask Deutsche Welle
to develop one. They are Germany's international broadcaster funded by the government.