At the bottom there is an option for forwarding a subdomain, which is quite straight forward. So I filled in the details: https://ibb.co/irmCKK
However, forwarding doesn't work and get the error message "The connection has timed out" when I try to visit the subdomain. I've updated the DNS about 40 hours ago. I've also tried a VPN in the US (since I'm in Europe myself, and I though maybe DNS servers in the US are updated sooner) with the same result.
I think the problem is that I only have a security certificate for the main domain and not for the subdomain... But I don't want to purchase a certificate for the subdomain. I just would want the browser not to automatically switch to https for the subdomain (while for the main domain it should continue to always switch to https).