Category-specific "new topic" email address/url in notification email?

My discourse instance has multiple categories, and I’d like user notifications for category A to have links/email addresses to start new topics in A, those for cat B to start new topics in B, etc.

I see how to create a category-specific “new topic” url or email address. And I’d be all set if I had just one category.

When I attempt to customize user_notifications.reply_by_email, my guessed interpolation keys %{category_id} and %{category} are rejected as invalid.

What can I do?