You might try a DNS Challenge if you can update a TXT record in your DNS
But, even if you get a cert it looks like only clients that have Javascript enabled would work. And, maybe clients that meet other criteria as infinitree look to be taking care to block requests.
The best solution is to have infinitree allow URI's for ACME HTTP Challenges. ACME is an industry standard. They could just allow URI's with /.well-known/acme-challenge/
You could refer them to this forum for help
Or, consider a hosting service that already support free certificates.