GERMANYCW_terrain is the name. It is case sensitive.
\Eagle Dynamics\DCS World Server\bin>DCS_updater.exe install GERMANYCW_terrain
For the record, this is the command for the new Marianas WWII terrain on the dedicated server.
\Eagle Dynamics\DCS World Server\bin>DCS_updater.exe install MARIANAISLANDSWWII_terrain
00000.289 INFO : DCS/2.9.17.11733 (x86_64; EN; WWII-ARMOUR,SUPERCARRIER,WORLD,CAUCASUS_terrain,MARIANAISLANDS_terrain,NEVADA_terrain,NORMANDY_terrain,MARIANAISLANDSWWII_terrain)