The short answer: You can't unless the client site allows you to do so. A typical way to accomplish measuring external conversions is to use a postback pixels. You can easily google how they work - in short you would require your client to send a http request to your tracking software on the form submit. A good way to do this in practice is to pran embedable form to your clients that already includes this feature and sends along a clientID with the request, so that you can easily see which client generates how many filled out forms.