Hi Abhi,
Instead of creating an external link, could you use a report link instead?
If you must create an external link, one way to get the external URL is by using the Email feature:
Open the target report and select File > Email. In the Email preview window that appears, you should see the full URL for the current report.
There is probably a better way to get the URL, but that is one way.
Let us know if this helps,
Sam