Hi Jerry,
Here are two options:
1. You can try the option on the following URL
http://mscrmmindfire.wordpress.com/2014/04/23/open-dialog-box-like-crm-2013/
You can create a web resource with iframe to your custom page
and
2. You can create a section on the form and put your external url in an iframe,
When the button is clicked just make the section visible.
Hope it helps.
Bojan