But, when we try to enter the same in the SharePoint navigation, it gives us an error.
In order to counter this, we found out a workaround. There are some separate pages for quick launch and top nav as below:
1. Quick Launch: http://'site url'
2. Top link bar: http://
Here your can enter any kind of url. We were able to open the page from our site in a dialog box. The OpenDialog function we wrote in a js file and referenced in the master page.