How to enable imagick support in CMS or website in Plesk?
It is not required to set anything on Plesk side.
imagick module can be used in CMS if it supports it. CMS should automatically detect that imagick is installed or some configuration can be done using additional scripts like WordPress - https://wordpress.org/plugins/imagemagick-engine/. For the support of imagick module contact the corresponding CMS technical support.
For the other websites, website code should be adjusted.
Note: More information about imagick usage can be found here: http://php.net/imagick