Because you cannot use the form as trigger the plugin is not done this way the only way is if you use on click event on the submit button. In your form add id to the submit button:
<form action="ajax.html" method="POST" target="_blank" class="">
<input id="111" name="a" type="hidden" value="something-else" />
<input type="submit" id="cboxFormButton" class="button" value="Test">
</form>
and in your script add this:
$("#cboxFormButton").click(function(e){
e.preventDefault();
// Call the colorbox link from here...
$.colorbox({href:"/echo/html/"});
// if you want to the form action url do this
// $.colorbox({href: $(this).closest('form').attr('action') });
});