First you need to log-in to your easyname Controlpanel, then click on [Website Builder] and after that on [Login].
Now you are on the Website Builder's interface, now just click on the [Burger-Menu] icon on the left side, after that on [Site Settings]. Under "Extended" you will find the button [Security]. Here is the slider to activate SSL called [Redirect to HTTPS], turn it on. Now you just need to save your settings with a click on [Save].
Hint: You can activate SSL only on a paid Website Builder.