Implement hook_field_widget_form_alter()
to alter form elements of any content entity form.
For example how to add a validate callback when you have third party settings stored in the field configuration:
/**
* Implements hook_field_widget_form_alter().
*/
function mymodule_field_widget_form_alter(&$element, FormStateInterface $form_state, $context) {
$field_definition = $context['items']->getFieldDefinition();
if ($field_definition instanceof \Drupal\field\FieldConfigInterface) {
$mymodule_settings = $field_definition->getThirdPartySettings('mymodule');
if (!empty($mymodule_settings)) {
$element['#mymodule_settings'] = $mymodule_settings;
$element['#element_validate'][] = 'mymodule_element_validate';
}
}
}