Error 400 - Ampersand (&) on URL parameter value in iis 7.5

Could not find the right solution during my web search so I am going to ask this question.

I have a url like this

http://blahsub.blahdomain.com/blahsite/blahservice.svc/blahhandler?param=12345&msg=value%26more+value

My problem is that when there is a request and the "msg" has a value with an ampersand(%26) it turns the rest of the url into part of another parameter like so:

 msg=value&more+value

and returning  error 400 as a bad request.

I have the URL Rewrite module installed.

How can I allow ampersands in parameter values?

There are plenty of articles online but the ones that I have looked at has not given me the solution that I need.  

I need to use the URL Rewrite in IIS to do this.

add a rule?

thanks for your help.