// This is a part of the meta box settings
'validation' => [
'rules' => [
'field_id' => [
'required' => true,
'minlength' => 7,
// More rules here
],
// Rules for other fields
],
'messages' => [
'field_id' => [
'required' => __( 'Password is required', 'text-domain' ),
],
],
],