Hi,
The terms and conditions are for the purchases. So the checkbox will appear during the checkout process if you have the "terms and conditions" view added to your checkout workflow in the HikaShop configuration.
If you want a privacy content checkbox on your registration forms, you can configure the privacy consent plugin of Joomla in the plugins manager. HikaShop's registration forms are compatible with it.
Otherwise, you can create a custom field of the table "user" and of the type "checkbox" to create your own checkbox.