Jasper,
Thanks for your question.
When the "Run in Terminal" option is enabled, pRun checks if the entered executable exists in the standard executable PATH of Puppy Linux:
(/bin, /usr/bin, /sbin, /usr/sbin, /usr/local/bin, /root/my-applications/bin, /usr/games).
If the executable does not exist, you will see the popup dialog informing you the search item is invalid or not found.
Your search item (nzbcheck --progress -s "...563:4:$$1" -i /root/E1Amigos.nzb) is a complex multi-parameter directive which is not a PATH executable, so the search will fail.
For such cases, pRun provides a button to quickly launch a terminal window where you can enter the full multi-parameter executable string.
I hope this helps.
PS - The next release of pRun will support your request for a resizable GUI.
PupMates
https://www.smokey01.com/radky/PupMates.html