Add fields to a search index programmatically

use Drupal\search_api\Entity\Index;
use Drupal\search_api\Item\Field;

/**
 * Add field to search API.
 */
function HOOK_update_N() {
  $index = Index::load('ID');

  /* Index the field. */
  $field = new Field($index, 'field_machine_name');
  $field->setType('text');
  $field->setBoost(1.5); /* Do not set boost if your field type is integer*/
  $field->setDatasourceId('datasourceId'); /* e.g. 'entity:commerce_product' */
  $field->setPropertyPath('field_machine_name');
  $field->setLabel('Field');
  $index->addField($field);

  $index->save();
}