It's automated via the containerise addon, which can match url regexes. Google Regex, which works for a bunch of G-suite stuff (Gmail, Docs etc)

@.*\.google\.com.*username@gmail\.com.*$

A bookmark link for Gmail that works with that regex would be:

https://mail google.com/u/?authuser=username@gmail.com

Edit: What? Of fuck off HN, how the fuck are you supposed to post unformatted text? Ok, got it. I'm not translating the horrible YouTube url though.