Adding target _blank to ajax form based on response
I believe the answer is, you can't.  Pop-up blocking is intended to prevent the browser from opening a new window or tab when the action was not initiated by the user (they click on a link).  There are several ways to open a new window or tab; however, unless you can truly trick the browser into thinking the user actually clicked it (no, $(a).click() will not fool it) or the user has pop-up blocking turned off, you will always have the pop-up blocker doing it's intended duty.

I'm sure you're advising the user that you intend to open another window for them, why not just provide the link upon successful form submission? Alternatively, you can do a delayed redirect such as is often seen on, I believe, phpBB login screens (the ones that say, "Thank you for logging in.  You will be redirected to XXX in 5 seconds").