Apply Linked Name to Document Script

Do you mean something like that?

// shows only one way, provided that your file names always follow the same rule
var aDoc = app.activeDocument;
var aTF = aDoc.textFrames.add();
// names should always follow the same rule:
var aCon = "Sticker_IF_115340-2-1_Just the head(s)_3x3_X1";
aTF.contents = aCon;
var helper = aTF.contents.split("_");

// output
alert(helper[2]+"_"+helper[5]);
// write old and new name in the text frame
aTF.contents = "old: " + aCon + " --> new: " + helper[2]+"_"+helper[5];

Here's a modified version that I think will match the old and the new file name. Let me know how it goes.

- Mark

function test() {

    var myItem = app.activeDocument.selection[0];

    if (
        myItem == undefined
        || myItem.constructor.name != 'PlacedItem' // this avoids getting RasterItems that have file
        || myItem.file == undefined
    ) {
        // be sure that a linked item (and not an embedded) is selected
        alert('Please select a PlacedItem and try again.');
        return;
    }

    var myItemBounds = myItem.visibleBounds;
    var myItemHeight = Math.abs(myItemBounds[3] - myItemBounds[1]);
    var aTF = app.activeDocument.textFrames.add();

    // decode URL removes the %20 and other symbols
    var fileName = decodeURIComponent(myItem.file.name)
        // and remove the file extension
        .replace(/\.[^\.]+$/, '');

    // order number is considered to be a number with 5 or more digits, a dash, a number, a dash, and a number
    var orderNumRegex = /\d{5,}-\d+-\d+/;
    var orderNum = fileName.match(orderNumRegex);
    if (orderNum == undefined) {
        alert('Could not find orderNum in "' + fileName + '"');
        return;
    }
    orderNum = orderNum[0];

    // a qty is an X or x and a number, but only if it is at the end of the filename
    var qtyRegex = /X\d+$/i;
    var qty = fileName.match(qtyRegex);
    if (qty == undefined) {
        alert('Could not find qty in "' + fileName + '"');
        return;
    }
    qty = qty[0]

    textContents = orderNum + "_" + qty;
    aTF.position = [myItemBounds[0], myItemBounds[1] - myItemHeight];
    aTF.contents = textContents; //add the textContents to the textFrame.
}

test();

Edit: added some explanatory comments.