Code Test Page

Code For Step 2:

id="mainform"
<input class="inf_field" id="inf_option_Checkbox" name="inf_option_Checkbox" type="checkbox" value="">
<input class="inf_field" id="email" name="email" type="text" value="">

Code For Step 3:

<script type="text/javascript">
jQuery("#mainform").submit(function() {
if (jQuery("#email").val() != "") {
return false;
}
jQuery("#inf_option_Checkbox").val("5642");
jQuery("#inf_option_Checkbox").prop("checked", true);
return true;
});
</script>

Code For Step 4:

.inf_field {
  display:none;
  Visibility:hidden;
}