You just use the
IATA codes for the destination train stations (just as you do in the USA -
United-Amtrak Codeshare FAQ - Including ZFV (Philadelphia))
AIRail Service is currently offered between Frankfurt (Main) Flughafen Fernbahnhof (the long-distance rail station at the Frankfurt International Airport) and Stuttgart Hauptbahnhof (the main railroad station in Stuttgart), Köln Hauptbahnhof (the main station in Cologne), Dusseldorf Hauptbahnhof and Dortmund.
It has been years but did ZWS: Stuttgart Hauptbahnhof, Germany regularly for a decade