$serialized_entity = json_encode([
    'title' => array(
      'value' => 'some_title',
    ),
    'type' => array(
      'target_id' => 'node_type',
    ),
    '_links' => array(
      'type' => array(
        'href' => $base_url . '/rest/type/node/node_type',
      ),
    ),
    'entity_reference_field' => array(
      'target_id' => $entity_id, // This does not work. Using a valid uid.
    ),
    'normal_field' => array(
      'value' => 'some_data',
    ),
]);

'entity_reference_field' => array(
  'target_id' => $entity_id,
),

'entity_reference_field' => array(
  array('target_id' => $entity_id),
),