auto add jnews_primary_category in classic editor

global $pagenow;
if ( $pagenow === 'post.php' || $pagenow === 'post-new.php' ) {
    add_action( 'save_post', 'add_jnews_primary_category', 99 );
    add_action( 'edit_post', 'add_jnews_primary_category', 99 );
}

function add_jnews_primary_category( $post_id ) {
    $cat_id = 5; /* change this  */
    update_post_meta( $post_id, 'jnews_primary_category', array( 'id' => (int) $cat_id ) );
}