Hi,

Best way is to use our Screen Customization plugin.

Haha :')

For the one who are looking for the same thing I found it by my self...
You need to go to SqlElement.php and in function getExtraRequiredFields add the desired field.

For example in my case when an Action status is changed to done I want the field idEfficiency to become required to I wrote :

if (!isset($statusObj) || $statusObj == null) {
    $statusObj = new Status($status);
}
if ($class == 'Action' && $statusObj->id && $statusObj->name == 'done') {
    $result['idEfficiency'] = 'required';
}

Still the Screen Customization plugin is the Best way to do it.
Otherwise you need to make a change on code every time you upgrade Projeqtor.