I have add add_rewrite_tag function in my theme function file & its working fine where I need it, But its show hello world blog post on other post and pages.
add_action( ‘init’, ‘custom_rewrite_rule’ );
add_rewrite_tag( ‘%prod_brand%’, ‘([^&]+)’ );
add_rewrite_tag( ‘%prod_series%’, ‘([^&]+)’ );
Read more here:: add_rewrite_tag show Hello World blog post in all pages