The web service will send a message to the page displayed in the iframe. The page should have it's own api to receive and process the request.
There is no way to pass a parameter into an iframe. There are just different pages and they can communicate in the ways how different websites are communicate.
This article might be of some help. Towards the bottom it outlines how to dynamically add an iframe on a tab and set the src of the iframe to a URL which includes a value from the record. You could use this same approach to create an iframe on the contact and set the src url including the contacts ID value.
