To add or edit an entry to your domain-wide filter table, log in to your mailcow UI as (domain) administrator and go to:
Configuration > Email Setup > Domains > Edit Domain > Spam Filter.
Be aware that a user can override this setting by setting their own blacklist and whitelist!
There is also a global filter table in
Configuration > Configuration & Details > Global filter maps to configure a server wide filter for multiple regex maps (todo: screenshots).