[BirdOPrey5]

Just to give a little more specifics on what ForumsMods said, that does appear to be a custom user profile field.

In Admin CP you'd go to "User Profile Fields" -> Add New User Profile Field.
Type would be Single Line Text box.

In the options for the field I would make it required, on registration and mark it as private and not searchable or shown on "member list."

Then whatever the user enters there only admins will be able to see by bringing up their profile in the Admin CP.

Honestly though I don't see why this is necessary, if they are going to remember the word they'll remember the email address they used to register and that is enough to have their password reset.