How to link to supplementary materials that aren't hosted within DLG?

e.g. Highlander film, links to

Finding Aid for the Ed Friend Visual Materials

Finding Aid for S. Ernest Vandiver, Jr. Papers

For example, in the "additional resources" section, we need to be able to link out to external things: https://dlg.usg.edu/collection/dlg_vang

Resources for collections form is here: https://dlgadmin.galileo.usg.edu/collections/1804/collection_resources