Am I able to populate an entity form text field with a value from a URL link on an external site
Yes. Checkout the Field default token module. It lets you substitute tokens in place of a field's default value. In your case you want to use [current-page:query:var] for your field's default value. This will let you send someone to the URL http://example.com/my_entity_form?var=DEFAULT which will pre populate the field with DEFAULT.