Using Javascript to define which field id?

I'd like to adapt a bit of code Dan Diebolt posted some time ago, because it's nearly what I want.
The code is this:

Button Code:
"javascript:" &
"var QBU_rid = '" & [Record ID#] & "';" &
"$.getScript(gReqAppDBID + '?a=dbpage&pagename=myscript.js');" &
"void(0);"

Javascript Page Code:
var apptoken = "your application token here";
$.ajaxSetup({data: {apptoken: apptoken}});

var reason = prompt("Enter your reason For Declining This Request: ", "");

var promise = $.get(gReqDBID,{
  act: "API_EditRecord",
  rid: QBU_rid,
  _fid_6: reason
});

$.when(promise).then(function(){
  document.location.href = gReqDBID + "?a=dr&rid=" + QBU_rid;
});

// This example code was originally created by Dan Diebolt. His contact information is 734-985-0721 and email address is dandiebolt@yahoo.com //
​

The code works as intended, of course, but I would like to adapt it by making the field ID number (6, in the above code) a variable instead. If I could do that, I could re-use the javascript page in multiple places. As is, I'd need a new page every time I want to use the code, just to change the field id.

Ideally, I'd like to pass the fid number from the formula-url button the user will click.

Thank you for the help!

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Graham Haskin
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