You state that this is an external website. If the website was within the same domain, you could use jquery's load method to do this
$('#iframediv').load('https://www.google.co.in #DIVNAME');
If the page is external, you won't be able to do that. You could look at the external page and see if there is a named anchor. If there is, you can use that anchor in the src of the iframe to cause the iframe to start out scrolled to that position. I don't think it will get you exactly what you want but it's the closest you will be able to get with the page being on an external domain.
<iframe src="http://externalsite.com/page.html#nameofanchor"/>