I dont think it can do that, as far as I can tell...

You can save a particular string to always be there. But you can't save "<NOTHING>". As well if you set a custom to a particular search string then the other features get disabled -- which is non-optimal.

I can see where you are coming from though: You want it so if some text is selected, that text is in the find-box, otherwise the find-box should be blank.