How to register custom types into Cmdr?

I’m trying to register a custom Type with Nexus Admin (uses Cmdr as a base, so I’m asking for Cmdr in general, too) and I’m having difficulties with formatting and figuring out which arguments to use.

I’m trying to make a command that loads maps, with a custom type for each map.

This is the code so far. (Code is in a ModuleScript, NexusAdminAPI is the same as Cmdr):

--[[
Note: Not all the code/variables are included in this code block. Only the essentials.
]]

local ValidMaps = {}
local ActiveMaps = {} -- not used in this codeblock but I just pasted it anyway

for _,model in MapsFolder:GetDescendants() do -- inserts the maps into the map table
	if model:IsA("Model") then
		table.insert(ValidMaps, model)
	end
end

NexusAdminAPI.Registry:RegisterType("TrainingMap", { -- trying to register the custom maps type.
	Validate = function(ValidMaps)
		 return ValidMaps, "Invalid map"
	end,
	
	Parse = function()
		return true
	end,
})```