Prefilling webform with item_id of a related field

Hello,
I am trying to build a dinamic link via calculation to build a url to a WebForm. As the article below explains I have to inform the item_id. Via calculation I just have @UniqueID, that is a sequential number (ex. 1,2,3,etc).
How to obtain the item_id via calculation?

post: https://help.podio.com/hc/en-us/articles/215541788-Prefilling-fields-in-webforms

thanks