It wasn't super obvious to me, but you have to use beta.fastmail.com for this to work. Confused me for a moment as you can complete the first step of enabling

It wasn't super obvious to me, but you have to use beta.fastmail.com for this to work. Confused me for a moment as you can complete the first step of enabling this (changing to New Rules) using the stable client. --- Oh, no, it's even more confusing than that. You enable the labels beta function on the beta website, but then it applies even on the main website.