Hi Jason,
many thanks! However, I tried everything I could think of and I can't get it to work. I replaced the form ID (in my case 259) and the field (in my case "Interested_in_Newsletter__c" ). I put the code in <script> brackets and put a custom html on the landing page.
Do you happen to have any idea where I got it wrong?
Thanks again!
<script type="text/javascript">
$('#mktForm_259').attr('onSubmit','return validate_form ( );');
function validate_form ( )
{
valid = true;
if ( document.mktForm_259.Interested_in_Newsletter__c == false )
{
alert ( "Please check the Terms & Conditions box." );
valid = false;
}
return valid;
}
</script>