How to add logged in username after WordPress URL?

You can try this shortode in this plugin file or you can paste this method to bottom of active theme’s functions.php. If you are new to WordPress and need a help you can visit here for steps to add code in function.

add_shortcode( 'my-awesome-button', 'wp_my_awesome_bytton' );
function wp_my_awesome_bytton( $atts ) {
    $button_html="";
    if ( is_user_logged_in() ) {
            $current_user = wp_get_current_user();
            $button_html="<a href="https://direktoriku.com/shopping/?user=" . esc_attr( $current_user->user_login ) . '"><button>Click me</button></a>';

    }
    return $button_html; 
}

// Use this shortcode anywhere in page/post content like this: [my-awesome-button] it will replace your shortcode to html code. More information about shortcode visit here.