dont use a popup,
you are already using ajax functions, so use ajax to put the form in a layer, and onclick change the z-index and visibility of the layer to put it on top of everything else, so the user has to complete the form and submit/close the layer to continue
the same functionality as the private message prompts on this forum
this is a scrap of
<button id="frorm" align="center" onClick="showIt();">Important Information</button><script type="text/javascript">
<!--
(document.getElementById) ? dom = true : dom = false;
function hideIt() {
if (dom) {document.getElementById("layerm1").style.visibility='hidden';}
if (document.layers) {document.layers["layerm1"].visibility='hide';} }
function showIt() {
if (dom) {document.getElementById("layerm1").style.visibility='visible';}
if (document.layers) {document.layers["layerm1"].visibility='show';} }
--></script><div id="layerm1" style="position:fixed; left:10px; top:30px; width:95%; visibility:hidden; padding:10px; margin:10px; background:#ececec; border: 1px solid black;">
AJAX FORM
<button id="form" onClick="hideIt();">Close</button></div>
that demonstrates the principle, but not the ajax needed to replace that TEXT with your form