How to call a popup-window from an external source

By using JavaScript from a UI event, like onClick, and I do recommend to use jQuery, see the next example:

/// <reference path="jquery-1.5.2-vsdoc.js" />
/// <reference path="jquery-ui-1.8.11.min.js" />
/// <reference path="jQuery.tmpl.js" />

window.onload = function () {
    $("#defineQuantity").dialog({
        autoOpen: false,
        title: resourcesTexts.defineQuantityTitle,
        height: 'auto',
        width: 580,
        modal: true,
        resizable: false,
        cache: false
    });

   };

function openDefineQuantity() {
    $("#defineQuantity").dialog("open");
}

So in your have UI event, for this example, you should invoke openDefineQuantity

And in your HTML page you should have a div that contains the HTML for your popup, into a div:
<div id="defineQuantity" style="display:none;"></div>