Lua actions are identified using an action type of Lua and setting the specific Lua function and extra parameters (if required) in the action arguments.  Any extra parameters will be transferred directly to the Lua function through first argument (a Lua table) and the data can be accessed by using the parameter name as the table index.

--- @param gre#context mapargs
function CBUserParameter( mapargs )
    local p = mapargs.paramter1
    print("my_lua_func was passed : ".. tostring(p))
end