How to join WhatsApp group from "chat?code=" link
The marketing link clicked in the original email "secretly" went through a http://chat.whatsapp.com/(randomlookingthing) link first, before landing on the whatsapp://... link.  That link is only understood by phones that have WhatsApp installed.  As noted, the Firefox on the computer doesn't understand the whatsapp:// protocol, and the browser on the phone doesn't either.  What is needed is the chat.whatsapp.com/... link.

The easy solution is to forward the original email to the phone.  That link, with the original marketing link, should bounce through the chat.whatsapp.com link, then, since WhatsApp is installed on the phone, it will understand how to add you to the group.

Alternatively, you can open Firefox's Developer &gt; Network, then run the marketing link.  This will expose the chat.whatsapp.com link that's aligned with the group.  If that link is used in the phone's browser, since WhatsApp is installed on the phone, it will understand how to add you to the group.