Message system has nothing to do with the file system.
The simplest ways to check what is the address of the script you send message to is to add the following line in init() function in the script:
print(msg.url())
It will be really hard for you to understand how things work if you will not spend some time for manuals reading which covers all the game engine systems.
This particular topic is well explained in this manual: Message passing in Defold
If you like video format more, then you can take a look (but I still recommend read manuals):
