Hi Enrico,
Attached is a little sample and a customization which should help you. I let you try it and let me know whether or not it corresponds to your need.
See a video of the result you should get with it:
https://www.screencast.com/t/iPoLSRtn . I tested with latest GBC 1.00.30 version.
If you are new to GBC customization, I invite you to follow our documentation. Here is the entry point
http://4js.com/online_documentation/fjs-gas-manual-html/#c_gwc_js_cust_intro.html .
Regards,
Guney