Use this syntax for the non-file parameter:
%"prompt"%
or
%(String)"prompt"%
(String) is optional, since quoted text is interpreted, by default, as string data. This syntax displays a dialog box that prompts for non-file data; do not use this syntax when prompting for a file name.
For example, this execution string runs the xwd command and prompts for a value to be added to each pixel:
EXEC_STRING xwd -add %"Add value:"% -out %Arg_1"Filename:"%