Add ID in custom links

Hello,

I would really need to be able to add the ID of a lead or a contact in custom links that one can create (Setup -> Customization -> Links).

I have been able to do that for a link in module Sales Orders: www.test.com?SO=${SalesOrders.SO Number} but am not able to do the same in module Leads and Contacts. Have been trying with: www.test.com?Lead=${Leads.LEADID} but I get a warning (Given field label does not exist).

Could you please add LEADID and CONTACTID in the drop down menu or at least not forbid to add them manually ?