Sorry this is currently not possible. This would have to be done by injecting an object in javascript in your HTML page and connecting it with the TTMSFMXWebBrowser. We will add this on our feature request list.
The current implementation of the TTMSFMXWebBrowser is for basic usage such as navigating webpages, loading HTML pages and executing javascript. There is currently no support for javascript to Delphi. We will further investigate this here and see if we can add support for this. There is currently no alternative.