Need URL variable to post into iFrame src

WPMU: I have a number of links on my site that link to an external website. I would like to instead link to a common page on my own site with an iFrame in it, and have the external URLs be added to the end of the common page URL as a query string, and to have that URL be posted into the iFrame src and displayed there.

I posted this at premium.wpmudev.org/forums/topic/how-do-i-embed-a-url-from-a-url-string-into-an-iframe?replies=8#post-656981 - but unfortunately, a final solution was not found.

Can you please help?

Thank you,
Brett Miller