Selected text is filled in in case it's selected up to one line only. If more than line is selected, then no text is filled. You can access last used text with Down arrow.
If you want to customize Search dialog behavior, open Program settings / Dialog. There is few options for it. -- <https://forum.pspad.com/read.php?2,79425,79426> PSPad freeware editor https://www.pspad.com