doesn't seem to work in slack,
also to the person asking to redirect youtube links as well
https://addons.mozilla.org/en-US/firefox/addon/redirector
with these setings:
name:
youtube.com => hooktube.com
Redirect:
*youtube.com/watch?v=*
to:
https://hooktube.com/watch?v=$2

It allows to watch videos, that youtube doesn't allow.
There are a lot of addons that promise that they unblock youtube - any of them does not.
This addon - allows! Thank you!