WordPress - Dynamically add user ID to the end of a URL

function get_user_specific_url() {
    if ( is_user_logged_in() ) {
        return sprintf(
            '%s/custom-post-type/%s',
            get_site_url(),
            wp_get_current_user()->user_nicename
        );
    }

    // Redirect home in case if not logged in
    return get_site_url();
}

add_action( 'init', function() {
       add_rewrite_tag("%user_nicename%", '([^/]*)');
       add_rewrite_rule(
           '^custom-post-type/([^/]*)/?',
           'index.php?page_id=123&user_nicename=$matches[1]',
           'top'
       );
} );

add_shortcode( 'replace_link_w_user', 'rl_w_user' );
function rl_w_user($atts = array()) {
    $details = shortcode_atts( array(
        "link"=>'',
        "button_text"=>'Search'
    ), $atts );
    if ( is_user_logged_in() ) {
        $button='
        <form action="'.$details['link'].'/'.wp_get_current_user()->user_nicename.'">
            <input type="submit" value="'.$details['button_text'].'" />
        </form>';
        return $button;
    } else {
       return;
    }
}