How to set prediacate for set-popup-rule?

What happened?

Doom docs show the syntax to set pop rules, but how to find the exact text that I should use for a particular window? I want to make lsp-treemacs-call-heirarchy open the popup on the right, so as per the docs, I should have the following in my config.el:

(after! treemacs
  (set-popup-rule! "^Call Hierarchy" :side 'right))

What did you expect to happen?

Popup window open to the right of the screen when I run lsp-treemacs-call-heirarchy. Instead it, open up on the bottom of the screen.

System information

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