if (arg(0) != 'node') {
  return;
}
$node = node_load(arg(1));
$tids = array();
$vid = 2; // this is the vocabulary id to search through...
$terms = taxonomy_node_get_terms_by_vocabulary($node, $vid);  
foreach($terms as $term){
	$tids[] = $term->tid;
	}
return implode('+',$tids);

Use this on a Term ID argument - enter the below as provide default agrument - php Code

remember to change the vid to your vocabulary

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