Can I open a new page when I select an item from a dropdown

I am working on a form where I want the user to select an item in a dropdown and that automatically opens a new page with an external URL.  

How would I go about doing this?

It is not currently possible pop-up a new page with an external URL when a selection is made within the form.  This would cause many security issues as it would allow users to force people filling out the form to visit malicious web pages.

You could embed your pages into the form itself using iFrame widgets:

http://widgets.jotform.com/widget/iframe_embed

Then conditionally show those frames based on the selection made in your dropdown field:

https://www.jotform.com/help/316-How-to-Show-or-Hide-Fields-Base-on-User-s-Answer

Or you can have a conditional thank you page based on the users selection:

https://www.jotform.com/help/317-Change-Thank-you-URL-Based-on-a-Condition

This will redirect them to your page after the form has been submitted.