"We want to run our landing page on https but we are unable to because we do not have Wishpond’s SSL certificate. It seems like it would only work on http."
Starting February 2018, Wishpond has started to provide SSL for publishing the Wishpond on https for custom domains.
The way you do this is identical to how you would publish your campaign on custom domain via CNAME record change. From your DNS CNAME record section, add a record that points your chosen subdomain to "tls.wishpond.com" instead of "wishpond.com" or "insecure.wishpond.com."
If you encounter problems with the SSL certificate failing with an error that says "certificate not trusted" or a similar message, make sure that you have added letsencrypt.org in the CAA DNS record for your top level domain. For information on how to add the CAA DNS record, please see https://letsencrypt.org/docs/caa/