How to set custom field value for programmatically created Relation entity

I am using the Relation module and need a custom date field one of my relation types. I added the field to the relation type, set the field as required and set the default value to 'now'. I am using a Rule to create the relation programmatically, with the code below:

 $endpoints = array(
    array('entity_type' => 'user', 'entity_id' => $account->uid),
    array('entity_type' => 'node', 'entity_id' => $session)
  );
  $log = relation_create ('has_logged', $endpoints);
  $log_rid = relation_save($log);

The relation is created and that works perfectly. However, the custom date field of the newly created relation is not set with the default value; it is empty. How do I set the default value of the date field?