To 'alter' what shows in an entity reference selection

Reference https://drupal.stackexchange.com/questions/298105/can-i-create-non-default-entityreferenceselection

In src/Plugin/EntityReferenceSelection

<?php

namespace Drupal\YOUR_MODULE\Plugin\EntityReferenceSelection;

use Drupal\node\Plugin\EntityReferenceSelection\NodeSelection;

/**
 * Provides a query for a node entity reference selection.
 *
 * @EntityReferenceSelection(
 *   id = "YOUR_MODULE_REFERENCE_SELECTION_ID",
 *   label = @Translation("Filter nodes with a specific field value."),
 *   entity_types = {"node"},
 *   group = "YOUR_MODULE_REFERENCE_SELECTION_GROUP",
 *   weight = 1
 * )
 */
class YourModuleEntityReferenceSelection extends NodeSelection {
  /**
   * {@inheritdoc}
   */
  protected function buildEntityQuery($match = NULL, $match_operator = 'CONTAINS') {
    $query = parent::buildEntityQuery($match, $match_operator);
    $query->condition('field_featured', 1', '=');
    return $query;
  }
}

When adding a form using Form API, best practice is to use dependency injection when possible. Admin form classes typically 'extend ConfigFormBase'. If you want to add to it, it'll look like this.

<?php

namespace Drupal\my_module\Form;

use Drupal\Core\Config\Config;
use Drupal\Core\Form\ConfigFormBase;
use Drupal\Core\Config\ConfigFactoryInterface;
use Drupal\Core\Messenger\MessengerInterface;
use Symfony\Component\DependencyInjection\ContainerInterface;

/**
 * Class myCustomAdminForm.
 *
 * @package Drupal\my_module\Form
 */
class myCustomAdminForm extends ConfigFormBase {
  
  /**
   * @var $configFactory
   */
  protected $configFactory;
  
  /**
   * @var Config $myConfig
   */
  protected $myConfig;

  /**
   * The Messenger service.
   *
   * @var \Drupal\Core\Messenger\MessengerInterface
   */
  protected $messenger;
  
  public function __construct(
    ConfigFactoryInterface $configFactory,
    MessengerInterface $messenger
  ) {
      parent::__construct($configFactory);
      $this->myConfig = $this->config('my_config.settings.my_settings');
      $this->messenger = $messenger;
  }
  
  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container) {
    return new static(
      $container->get('config.factory'),
      $container->get('messenger')
    );
  }
  
    /**
   * {@inheritdoc}
   */
  protected function getEditableConfigNames() {
    return [
      'my_config.settings.my_settings',
    ];
  }
  
  /**
   * This needs to return a string that is the unique ID of your form. Namespace the form ID based on your module's name.
   */
  public function getFormId() {
    return 'my_module_admin_form';
  }
  
  /**
   * This returns a Form API array that defines each of the elements your form is composed of.
   */
  public function buildForm(array $form, FormStateInterface $form_state) {
  
  	$form['text_input'] = [
  		'#type' => 'textfield',
  		'#description' => t('Enter data into this field'),
  		'#default_value' => $this->myConfig->get('my_config_values')
  	]
  	
  	return parent::buildForm($form, $form_state);
  
  }
	
  /**
   * Validate the form
   */  
  public function validateForm(array &$form, FormStateInterface $form_state) {
  	$values = $form_state->getValues();
  	if(empty($values['text_input']) {
  		$form_state->setErrorByName('text_input', t('You must enter data into this field.'));
  	}

  }
  
  /**
   * Submit the form
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
  	$values = $form_state->getValues();
  	
  	$this->myConfig->set('my_config_values', $values['text_input']);
  	
  	$this->myConfig->save();
  	
  	$this->messenger->addStatus('Your value has been saved.');
  	
  	parent::submitForm($form, $form_state);
  }

}

Often times when using the Drupal Form API creating custom forms, you'll need to get a select list of Taxonomy terms as options. These snippets will help you look up terms in a vocabulary and put them in your options array.

Drupal 8

// Create empty options array
$options = array();

// Taxonomy vocabulary machine name
$taxonomy = 'tax_machine_name';

// Get all the taxonomy terms from the vocabulary
$tax_items = \Drupal::entityTypeManager()->getStorage('taxonomy_term')->loadTree($taxonomy);

// Inject into the options array
foreach($tax_items as $tax_item) {
  $options[$tax_item->tid] = $tax_item->name;
}

// Create a select form element
$form['taxonomy_options'] = array(
    '#type' => 'select',
    '#options' => $options,
    '#required' => TRUE,
);

Drupal 7

// Create empty options array
$options = array();

// Taxonomy vocabulary machine name
$taxonomy = 'tax_machine_name';

// Get all the taxonomy terms from the vocabulary
$vocab = taxonomy_vocabulary_machine_name_load('tax_machine_name')->vid;
$tax_items = taxonomy_get_tree($vocab);

// Inject into the options array
foreach($tax_items as $tax_item) {
  $options[$tax_item->tid] = $tax_item->name;
}

// Create a select form element
$form['taxonomy_options'] = array(
    '#type' => 'select',
    '#options' => $options,
    '#required' => TRUE,
);