Create a JS function to read document.location.href and send this in the flashVars parameter of your <embed>
In your Flex code, you'll have the value you sent in Application.application.parameters (i.e. if you send it as "currentLocation", you'll have it as Application.application.parameters.currentLocation)
After checking if you need to redirect, do either of the following to send the user to the new URL:
flash.external.ExternalInterface.call("window.open", "https://www.mysite.com", "_self");
var request:URLRequest = new URLRequest("https://www.mysite.com");
If you want all visits to your site (or even a particular URL) to be https, this is probably better done through server-side redirection.