In Event Setup - Modules you can now setup a desktop module which will link to a customized url.

Module name is "Custom service"

Here an example, important the url must start with http:// or https://:

Here how it looks in your event. On click this module will open the url in a new tab.

After creating one module, you can change the title or translate in other languages in Event Setup - Ui Texts - UI group "Desktop" - "Name of the created module".