Hey really recommend using a big long random string in that URL, because as you will have read above TAILNET NAMES ARE PUBLIC. You can find them here: https://crt.sh/?Identity=ts.net [warning, this will probably crash browser if you leave it open too long -- but you can see it's full of tailnet domains].
So anyway try it like:
tailscale funnel --set-path=/A8200B0F-6E0E-4FE2-9135-8A440DB9469D http://127.0.0.1:8001 or whatever
I use uuidgen and voila.