$link_data = new AdCreativeLinkData();
$link_data->setData(array(
  AdCreativeLinkDataFields::MESSAGE => 'try it out',
  AdCreativeLinkDataFields::LINK => '<URL>',
  AdCreativeLinkDataFields::IMAGE_HASH => '<IMAGE_HASH>',
));

$object_story_spec = new AdCreativeObjectStorySpec();
$object_story_spec->setData(array(
  AdCreativeObjectStorySpecFields::PAGE_ID => <PAGE_ID>,
  AdCreativeObjectStorySpecFields::LINK_DATA => $link_data,
));