In case you wish to have an extensive list of access codes, you can use a Data List of access codes to serve as a whitelist. The procedure of creating a list is the same as making an Email List: In section Email => Lists, you can add a new list, and while adding it, mark it as a Data instead of Email (type of) list. The Data List has only one field (name) instead of two (email too). You can add custom fields that will act as other columns of that list. Once you have created a Data List, you can go back to Survey Options page, and instead of typing in the access codes in the text field, you can simply link this Data List (and it's column that will serve as access code) in a dropdown underneath.
If your survey is connected to a Data List, you will also be able to use personalization tags in your Free Form HTML question types. There you will be able to use person's data in the middle of the question, which can make your surveys look far more user friendly.
