Hi @PompousCat and welcome to the forum,
(FYI I edited the name of your post; hopefully that’s approx. what you were wanting to put.)
It can’t be done with an Asana rule, as Asana rules don’t run on subtasks.
Two suggestions I have:
You could do it in Zapier. However, if you’re using the “Create subtasks in Asana” action as I’m guessing you are, that action doesn’t support assigning a project I don’t think. You could do it by using Zapier webhooks (which is basically a lower-level way of doing something). That’s more complicated than using a regular action but should work. You can find some info on writing to Asana via Zapier webhooks in the Developers & API section of the forum here, for example: Automatically create tags from external integration
Another option is to use our Flowsana integration, as our rules can be set to apply to subtasks. Basically, in Project A you would create a rule of “When a new task is added, Add it to Project B” and you would set that rule to run for Subtasks only. That would do what you’re wanting, it seems.