I tried, without extensive registry-fu it's pretty much impossible now. In the name of making it easier for John and Jane Doe, extended association functionality went downhill beginning with Vista (along with the disappearance of the tab in Folder options). They may have restricted it even further or may have removed that functionality altogether because it clashes with ''Modern'' UI.
In case you want to try, the registry key should be
HKEY_CLASSES_ROOT\ftp
In case you want to try, the registry key should be
HKEY_CLASSES_ROOT\ftp