After the last line of any FORM you add the following code

[PHP]<?php $code=""; for ($i=0;$i<=6;$i++){ $code.=chr(rand(65,90));} ?>
<tr><td class="enquiry">Enter Code:&nbsp;&nbsp;<span style="background: red; padding: 5px; font-size: 14pt;"><?php echo $code; ?></span></td><td><input type="text" name="code" value="" />&nbsp;(case sensitive)<input type="hidden" name="code2" value="<?php echo $code;?>" /></td></tr>[/PHP]

That will add the code and the fie.d to enter it. In the validation code you then just compare the two variables like this 
[PHP]if (trim( mosGetParam( $_POST, 'code', '' ) ) <> trim( mosGetParam( $_POST, 'code2', '' ) ) ) { display error message }
[/PHP]

I hope this helps.