Adding elements to a field widget in Drupal 8 - drupal

Question

What I need to accomplish is adding a link to a node to field widgets (the node contains the formatted instructions to properly compile the field).
So far I've been able to add the node reference field in the field config form, using Third Party Settings:
function mymodule_form_alter(&$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id) {
if ($form_id === 'field_config_edit_form' && $form_state->getFormObject()->getEntity()->get('entity_type') == 'myentity') {
$field = $form_state->getFormObject()->getEntity();
$help_page_id = $field->getThirdPartySetting('mymodule', 'help_page_id');
$form['help_page_id'] = array(
'#type' => 'entity_autocomplete',
'#title' => t('Help page'),
'#target_type' => 'node',
'#selection_handler' => 'default',
'#selection_settings' => array(
'target_bundles' => array('help_page'),
),
'#default_value' => $help_page_id ? Node::load($help_page_id) : NULL,
'#weight' => 100,
);
$form['#entity_builders'][] = 'mymodule_form_field_config_edit_form_builder';
}
}
Now I'm having troubles retrieving this information when showing the form, and altering the field widget in order to display the link.
Using hook_field_widget_form_alter, I cannot get the custom configuration value from the arguments I have:
function mymodule_field_widget_form_alter(&$element, \Drupal\Core\Form\FormStateInterface $form_state, $context) {
}
I guess I have to load the field configuration entity, but I don't know how.

Answer 1

I didn't test it, but something like this should work:
function mymodule_field_widget_form_alter(&$element, \Drupal\Core\Form\FormStateInterface $form_state, $context) {
// Add a suffix to widget form elements for all fields of type entity_reference.
$field_definition = $context['items']->getFieldDefinition();
if ($field_definition->getType() == 'entity_reference') {
// TODO: Render the node link to $link
$element['#suffix'] = $link;
}
}