I don't think the register_token field is what you want to use, that is a field used by the system . Are you just looking for the user to supply some sort of predefined "code word" to be granted access?
Try creating a new field and adding it to your form:
Field Type: string
Name: code_word
User interface: Text Field
Label: Enter Secret Code
Field Required: yes
Validator: IsEqual
Validator Argument: "your code word"