add &vericode=`1` in your snippet call
In the chunk add something like the following:
<p>
Please enter the anti spam code below:
<img src="[+verimageurl+]" alt="verification code" border="1"/>
</p>
<p></p>
<p>
<label accesskey="c" for="vericode">code</label>
<input type="text" name="vericode" size="20" />
</p>
--[/center]
