It should be something like below
<br><br>
function mymodule_civicrm_validateForm($formName, &$fields, &$files, &$form, &$errors) {<br>
  if ('CRM_Event_Form_ManageEvent_EventInfo' == $formName) {<br>
    if ( validation condition fails )<br>
      $errors['custom_element'] = ts('Validation message');<br>
    }<br>
  }<br>
}<br>
<br>
HTH<br>
Pradeep