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I have created and installed a cert for my wildcard domain *.example.com OK using a dns challenge. So far, so good.
Now I want to renew the cert using a cronjob. I will need to use the http challenge because my DNS host has no API mechanism for me to automatically create the TXT record. What I don't understand is how to tell certbot/letsencrypt where my http server is, given the domain is a wildcard that doesn't point to the server where I'm running certbot.
What you wanting is not possible. As per https://letsencrypt.org/docs/faq/#does-let-s-encrypt-issue-wildcard-certificates wildcard certs must be done via a DNS challenge.
This is also restated - along with DNS provider hints at https://letsencrypt.org/docs/challenge-types/
If you think about it this makes sense. To allow otherwise would enable issuing certs where the authority of the requestor is in doubt. (Ie someone who controls the TLD web site would be able to get a wildcard cert for subdomains which they may not be authorised to). On the flip side, someone who controlsndns for the domain automatically has the power to direct subdomains.
