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What you can do is rename it to a more appropriate field, let's say "Unique Access Code" or "Authorization Code", something that doesn't resemble any sensitive information. Here's a sample form that I made: http://form.jotformpro.com/form/50965918005964.
Here are the steps on how I made it:
1. Add a plain text box. Setup Conditions by clicking the its icon.
2. In the conditions wizard, choose "Show / Hide a form field", to learn more about Conditions you can check this guide: http://www.jotform.com/help/57-Smart-Forms-Using-Conditional-Logic.
3. This is the rule and action that I setup. If the "Enter Code" field is equal to "1234" then show all the form fields including the submit button. If this is not meant it automatically hides the said fields.
I hope that helps.