I'm not sure about what you're trying to accomplish, but in script-fu you can create a dialog box to accept user input. Items in the dialog box can be specified and customized in the script-fu-register section at the end of the script. Here
is a good example with an explanation of the available keywords. Each of these selections will be an argument to the main function. For example:
(define (script-fu-parrot inString)
; pop-up a message with the string typed by the user
"This procedure echoes a user defined string and exit"
"May 16, 2010"
SF-STRING "Type something" ""
Here the value entered as SF-STRING in the dialog box is passed to the function as the first (and only) argument "inString". Is this what you mean?