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Secure custom fields can be created by JIRA Admins while being in the View Issue screen with the Admin > Add field > Advanced optionoption.
Simply begin typing a new name and "(Create new field)" option should appear. After clicking "Create field" navigate to "Advanced" tab and type "secure" into the search bar in the upper right corner or just look for the Secure Field type that's interesting you. Note that you do not have to leave the screens when you want to configure the options for the chosen custom field.
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