Although I can't find this in the release notes or documentation for FM Cloud 18.104.22.168 or FMP 16 or FMPA 16.
I found that the Cards window feature works with FM Cloud 22.214.171.124 with the following clients:
FMPA 16.04.403 (windows)
FMP 16.04.403 (windows)
FMPA 16.05 (Mac)
FMP 16.04 (Mac)
FMPA 15.x (Mac)
Apparently the Cards window functionality is based solely on the client and works independently of the server.
I'm not sure this is news to anyone except me but for some reason I thought I needed to upgrade the FM Cloud to 1.16 and use FMP 16.x to use the card functionality. This is a pleasant surprise because I have clients running both FMP 15 and 16 due to hardware limitations on the Mac side. Simple enough to query the client version to route the script to avoid the card function on FMP 15.