When you install a managed package in your Salesforce org with Shield Platform Encryption, you can encrypt custom fields in the package. You can encrypt any field that belongs to one of the encryptable custom field types.
Engage your app vendor to make sure they are prepared to support encrypted custom fields in their package. Then contact Salesforce support to enable this capability for your org.
As an admin, this may increase the range of packages you can leverage, because more packages will be encryption-ready out of the box. (Not all of them, though! Check with the app vendor that a package is compatible with encryption before you install it.)
You can encrypt the contents of fields that belong to one these custom field types, on either standard or custom objects.
- Text Area
- Text Area (Long)
It looks like the app has to support it and you need to contact SF support to enable it (See above):