All templates, messages and pages are fully HTTPS in Spotler by default. You don't have to adjust anything for this.
Have you worked with MailPlus before?
If so, it is possible that certain webpages and (campaign) messages are still partially HTTP, for example, because your images are not HTTPS. If the recipient is redirected to a non-secure environment, they will receive a security warning in the browser (this applies to webpages or online version).
Forms from Spotler that you have placed on webpages are always HTTPS. If you want to know if a template, page or message is HTTP or HTTPS, contact your Spotler partner.