MBWD on "[Plugin: WordPress MU Domain Mapping] Is it required to map only to https external domains?"

I am trying to map an http domain to an https subdomain of my multisite. This plugin is installed and activated, and in the dashboard of the subdomain in question I am adding the name of the exteal domain I want to map. The empty field shows http, but when I enter the domain and it appears in my list of available domains to make primary, it's shown as https. Obviously the exteal http domain doesn't have a cert on it, and so this is resulting in a security waing when you try to visit it.

I understand how to deal with this one particular instance--just slap a cert on the exteal domain. What I want to know is, how do I deal with this on an ongoing basis, if I have lots of domains I want to map? Do they all have to be https?

I am forcing HTTPS for this network with a page rule in Cloudflare, but I am loathe to disable it. If using https on exteal domains is a requirement for mapping to secure subdomains, I'm fine with that. I just haven't seen that actually set down anywhere as a rule, so I guess I am asking to have that clarified.

Thanks for a great plugin.

https://wordpress.org/plugins/wordpress-mu-domain-mapping/