In multicam, mc:cmd/mc:cmdwait accept named commands that are defined in the multicam script. That's what the 'unknown' in your example is about.
To run lua code, you have to use the call or pcall commands like
!return mc:cmdwait('call return get_focus()')
Return values from you code will be returned in the inner status member as an array.
You can see the pre-defined commands at the bottom of the multicam script, around
local function init()
