Sometimes we have a "HiddenType" field in a Symfony 3 form and this field is completed with a step outside the form (example: a service that allows you to choose a reseller on a Google map).
We would like to have a validation on this field and we also use the jquery-validate library. Here's how to proceed.

To begin, you must add the validator on the field in the buildForm() method in your class. We want it to be mandatory:

public function buildForm(FormBuilderInterface $builder, array $options) {

    $builder
      ->add(...)
      ...
      ->add('dealer_code', HiddenType::class, array(
        'required' => true,
      ));
  }

In your Twig view, it will be necessary to display the validation message where you want it to appear:

<div class="input-field col-md-12">
            <span>Select a dealer</span>
            {{ form_widget(form.selectionner) }} {# a button that leads us to our reseller location service #}
            {{ form_errors(form.dealer_code) }} {# we decide to display the error message here otherwise this message will be displayed below all fields of the form #}
            {{ form_widget(form.dealer_code) }}
</div>

At the level of jquery-validate it will be necessary to add the option "ignore" with an empty array otherwise the validator on the fields of type "hidden" will be ignored:

$("#formValidate").validate({
        ignore: [],
        ...,
        messages: {
             dealer_code: {
                  required: "Please choose a reseller"
             }
        },
});