It's not possible to create a link to a custom route. In the link modal:

1. Entering "link" sends "http://link" to the back-end
2. Entering "/link" sends "http:///link" to the back-end

New editor should behave in the same way as the old one (alloy) in 3.3:

1. Entering "link" sends "http://link" to the back-end
2. Entering "/link" sends "/link" to the back-end