Trac ticket-custom permission

I created a custom field checkbox in trac. Written here.

How to make the field were only available for users to TRAC_ADMIN?

example trac.ini

[ticket-custom]
newfield = checkbox
newfield.label = Checkbox field name
newfield.value = 0
newfield.permissions = TRAC_ADMIN

newfield.permissions - does not work

Thank you.