Having had a look at the activity feed on that board, everyone is added by the admin, although it's a public board, there's no way to join it unless you're added, you can only see it as a non-member.
I would try and contact the company through a different medium and see if you have more luck there, or if they have instructions on their application method.
If you had to thrive a new habit during a lockdown, what would it be? Trello
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