Your solution is great but will not work for him (He want 3internal pages and one extrnal “new window” page). Of course manually he could write the link for each page (But this is not DRY).
@helen880201 the external link should open in new window?
Why not creating two collection (and use limit or “filter”).
Create Switch field - link type (internal & external ) - Create two lists (under webflow tree = HTML) - one "show only if “link type” is set. And the other the opposite.
Another option is to filter by “External URL” is set (In my opnion its better to create logical feild).
Summary: One colletcion - one radio button - two collection list under webflow tree (HTML) - done.