When inserting a link, populate both fields with the entered text, which can be left as-is, or edited to be different, as in gmail.
It is unnecessary to leave the link field empty when inserting a link. Duplicate the text that is "linkified" into the link field. Thus, when a link is highlighted to be an inserted link, populate the link field with the same as the text field, and allow editing in case the user wishes a difference between the highlighted text and the link. Gmail does this and it often saves a step.