Discourse’s internal Permalinks can do this for you.
You’d designate a URL like 02 and set the permalink to point to whatever category or topic that ends up corresponding to. (It might be category 7, or topic 42, etc.)
Example: I have a topic https://forum.tasat.org/t/stories-about-blocking-out-the-sun/505
I could set a permalink for /sunblock that points to that topic:
…and share the link forum.tasat.org/sunblock