Hier das Codeschnipsel für die Nachwelt, falls es jemand braucht.
export function Schaltfläche_1_OnDown(item, x, y, modifiers, trigger) {
let ui = UI;
let count = ui.PopupScreenWindows.Count;
let pos_x = 550;
let pos_y = 310;
// check if there is already an pop up openned, save his position...