"Webform Promotion Code" will expose a new field type called "Promotion code". This "Promotion code" will be rendered as a text field for your users. The user will use this field to insert a "Promotion code". When the user submits the form, the information inserted on this field will be validated against a code list registered by Drupal admin when the Webform was configured (see the screenshots below for more details).
You can use "Webform Promotion Code" in order to add passwords to your Webforms.