Reference current user when creating a content type I am creating a new content type and one of the fields I d like to add owner. 

I ve chosen reference - user as the field type 
Type of item to reference = user

How do I make the default value the name of the current user?

Many Thanks
You can set default values in a hook_ENTITY_TYPE_create:

/**
 * Implements hook_ENTITY_TYPE_create() for 'node'.
 */
function mymodule_node_create(\Drupal\node\NodeInterface $node) {
  if ($node-&gt;getType() == 'my_content_type') {
    $node-&gt;field_user-&gt;target_id = \Drupal::currentUser()-&gt;id();
  }
}