[Question] Uncaught Shadow pokemon going to specific trainer

I'm slowly getting back into my pokemon fan game. A major plot point of the game involves heavy use of shadow pokemon. In pokemon xd, if the player fails to catch a shadow pokemon, it would go to miror b to be caught at a later time.

I want to do the same thing, but using a different character. The problem I'm having is there is no place I can find that tells me how to do this. The essentials wiki shows this blurb when mentioning the topic:

In Pokémon XD, Miror B. would collect any Shadow Pokémon the player knocked out, and would occasionally appear with these Pokémon. You could add a similar feature to your game, using the array $Trainer.shadowcaught to determine which Shadow Pokémon Miror B. can have.

Click to expand...

And that's it. Nothing about how to use the array to achieve what XD did, or anything. I'm sorry if I'm missing something, but how am I supposed to know what to do from just this?