Assign a tag to custom post type using a query

Put code below to your functions.php

add_filter(‘pre_get_posts’, ‘query_post_type’);
function query_post_type($query) {
if(is_category() || is_tag()) {
$post_type = get_query_var(‘post_type’);
if($post_type)
$post_type = $post_type;
else
$post_type = array(‘post’,’cpt’); // replace cpt to your custom post type
$query->set(‘post_type’,$post_type);
return $query;
}
}