Director: Custom constant in command field within command definition

I’ve defined

const Vars = { MyPluginDir = “/usr/local/libexec/myplugins” }

How can I use this variable in the “command” field of a command definition?
Vars.MyPluginDir is always treated as a string.

I have tried:
/$Vars.MyPluginDir$ + mail/check_email
“/” + Vars.MyPluginDir + “mail/check_email”
Vars.PluginDir + mail/check_email