How to refer User-defined variables in Custom JavaScript

I create a user-defined variable var1, and I’d like to use it in another user-defined variable (Custom JavaScript) var2.
How should I refer var1 in the code of var2?
I try to use {{var1}} but it won’t work.

And I can’t find ways to call it in the object MatomoTagManager.

I mean, I can see the variable I want in it but it cannot be called anyway.

Does anybody know about this? Or is there other ways to do this?
Thank you so much.