Template variables and buttons in Screen designer for Custom Objects

Hi,

In screen designer we can add buttons on a custom object.

My customer want to be able to open an external url (sharepoint) related to the chosen id. The URL is also saved in an extrafield on the custom object.

Since it's an external URL I guess we can't use EventData and setNavigateTo, so I'll try to use "Open webpage" on the button and create a script that opens a URL with a cgiVariable. This works fine on tickets, but I can't figure out how we can take the custom object id from the URL and add it as a cgiVariable when running a script?