[uberchel]

Повесить событие onsubmit на нужную форму.

<iframe name="blank" width="300" height="250"></iframe>
<br>
<form id="form" action="http://game-of-thrones.su/" method="post" target="_self">
<input type="submit" name="sbm" value="go">
</form>
<script>
var form = document.getElementById('form');
if (form.addEventListener) {
    form.addEventListener('submit', function () {
       this.target = 'blank';
    });
} else {
    form.onsubmit = function () {
       form.target = 'blank';
    }
}
</script>