Show Sitename on Yoast SEO Title tag [closed]

You can use the wpseo_title filter to read the tag for your site title after WPSEO creates it, and if it’s not there, add it on.

add_filter( 'wpseo_title', 'wpse137502_wpseo_title' );
function wpse137502_wpseo_title( $title ) {
    $site_title = get_bloginfo( 'name' );
    if ( ! strpos( $title, $site_title ) ) {
        $title .= " | " . $site_title;
    }
    return $title;
}