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 - but unfortunately, a final solution was not found.

Can you please help?

Thank you,
Brett Miller