add field to bundle through hook_entity_bundle_field_info
This code example would set up a basic storage definition:

function mymodule_entity_field_storage_info(\Drupal\Core\Entity\EntityTypeInterface $entity_type) {
  if ($entity_type-&gt;id() == 'node') {
    $definitions['my_bundle_field'] = \Drupal\Core\Field\BaseFieldDefinition::create('string')
      -&gt;setName('my_bundle_field')
      -&gt;setLabel(t('My new bundle field'))
      -&gt;setTargetEntityTypeId($entity_type-&gt;id());
    return $definitions;
  }
}


You can look up examples in field.storage.*.yml files from a config export.

If you don't find the right examples there, you can configure a field in UI and then export the storage of this field to get a blueprint for the storage definition you need in this hook.