It's not what you wrote
Window.showModalDialog()
. JavaScript is case-sensitive, so this what you wrote is wrong function name; besides this function has been deprecated:
Window.showModalDialog() - Web APIs | MDN[
^].
Of course you can use
window.open
. I have no idea how it can be related to postback. Those functions are JavaScript functions, not related to server side activity at all.
However, using popup, in this sense of this word, is a bad idea. Not only it is inconvenient enough, but it is often blocked on the client side. The browsers' plug-ins blocking pop-ups are very popular, mostly because of annoying ads contaminating Web greatly.
In my article
Modal Popup From Scratch[
^] I discuss some better alternatives.
—SA