When I put an ID in to the ID field, and then try to link to it, it should JUST WORK. Stop making us have to research for obscure syntax, without even giving us a clue as to why linking is failing.
If you need to make a distinction between internal and external linking:
Put a dropmenu at the Link To line to choose internal or external.
Stop requiring us to know this:
Link to item within checklist: #item-id
Link to external checklist: checklist-id
Deep-link to external checklist and item: checklist-id;#item-id