It would be something like this:
<input type="text" name="Zip_Code" id="zip" />
It would be good to have a reset button for the captcha numbers (image) and I'm sure it's easy; but I don't yet know how exactly to do this - so I'm glad you asked - hopefully someone will post the answer.
I have tried putting a 'Reset' button on my form which works OK for resetting the values entered 'into the page (see my example here), but this doesn't reset the captcha image. The button somehow needs to reload the: 'WA_ValidationToolkit/WAVT_CaptchaSecurityImages.php' page.
A simple way would be to place a simple link on the form which links to the same page it's on:
<a href="contactus.php">Reset the Code</a>
This would reload the page, and therefore the captcha image as well. Not the most elegant solution, but it will 'work' and someone will hopefully provide the proper answer!