Add a field to a Drupal 8 entity.

hook_entity_base_field_info_example

<?php

use Drupal\Core\Entity\EntityTypeInterface;
use Drupal\link\LinkItemInterface;

/**
 * Implements hook_entity_base_field_info().
 * In this example, you get the id of an existing entity, then add a field to it.
 * Here, we create a link field that allows for link text, and then can be 
 * viewable on the output. 
 * 
 * This provides a handy way to alter existing modules, for example, without the need for a patch.
 * Cool stuff.
 */
function example_entity_base_field_info(EntityTypeInterface $entity_type) {
  if ($entity_type->id() == 'some_existing_entity_id') {
    $fields = [];
    $fields['link'] = BaseFieldDefinition::create('link')
      ->setLabel(t('Learn More Link'))
      ->setDescription(t('The location the Learn More link points to.'))
      ->setRevisionable(TRUE)
      ->setRequired(TRUE)
      ->setSettings([
        'link_type' => LinkItemInterface::LINK_GENERIC,
        'title' => DRUPAL_REQUIRED,
      ])
      ->setDisplayOptions('form', [
        'type' => 'link_default',
        'weight' => -2,
      ])
      ->setDisplayOptions('view', [
        'type' => 'link',
        'weight' => 0,
        'label' => 'hidden'
      ]);

    return $fields;
  }
}