HTTPS for logins can be enabled by an administrator in Settings > Site administration > Security > HTTP security.
HTTPS for logins (deprecated - don't use)
HTTPS encrypts the user's login data, so it's difficult to sniff out a user's username and password on the network. You will need to enable HTTPS on your server before you turn on this setting, or else you will be locked out of your site. Every web server has a different method for enabling HTTPS, so you should check the documentation for your web server.
It is not recommended that you use this setting as it's not secure despite using HTTPS for logins. Instead, please look at Transitioning to HTTPS completely
You are also unable to use the SSL proxy setting at the same time
It is recommended to use secure cookies only when serving over SSL. When not serving over SSL, the setting is ignored. In Moodle 3.1.2 onwards, the 'Secure cookies only' default setting is on. (It is likely that the setting will be removed in 3.2 onwards - see MDL-55662.)