How to get full URL in Frame

My client has an odd setup where his web site is hosted on an external
server but to enable access to it using his servers domain name/url a frame
is used pointing to the IP address of the external server. This works fine
for the top level (first link) with the HTML code below. However, a sub
page cannot be accessed directly this way. I'm looking for a way to get the
full URL passed to the frame and putting it into a variable that I can then
use with the src. This is on a Linux server running Apache.
I'd appreciate any ideas on this.

http://mywebsite.com/
http://mywebsite.com/susiepage

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