In the project settings you add the https:// but I actually leave it blank because it is not essential for this purpose.
The https part is actually controlled at the server end and you need to deploy an SSL certificate. Many web hosts provide this for free through Lets Encrypt these days. After that I would suggest you set up a 301 redirect inside the .htaccess that forces all traffic towards the https url. Something like I detailed here Change to HTTPS - #2 by Flashman