Currently there is no way to specify https in the canonical tag or sitemap when Strikingly is serving http and reverse proxied by Cloudflare or other https enabled proxy.
This affects SEO and things like Google Webmaster Tool not believing that you're authoritative or that the visited URL is canonical.
This can be enabled by contacting support but not via the user GUI or dashboard. Not sure of the percentage user base that needs this but also not sure of the percentage of user base who would notice. Happy to be wrong on this.