You are able to set a password within the Web Filter application to unblock websites (either temporarily or permanently, at your option). This setting can be found in Apps > Web Filter > Advanced:
The 'temporary' setting allows you to set a timeout after which the page will become re-blocked.
The 'require Password' option will prompt for a password: either the password for the NGFW's Administrator account or one you define in the 'Custom Password' field. If you do not require a password, a user will simply click a button to unblock that page. (In that case, we recommend setting a temporary timeout.)