Decode Querystring before ASM inspection

I still have not managed to resolve this, one - tryed loads of things nothing worked

The request URL is filename.aspx?parametername%3Dfakedata - the ASM see's all of this as one paramter name so thinks the parameter name is 'parametername=fakedata' rather than the parameter name being 'parametername' and the value being 'fakedata'. I am assumming this is because it is encoded

The ASM see it like this filename.aspx?parametername=fakedata

I tryed adding 'parametername=' as the parameter name, it excepts it as a parameter but does not match the with the above querystring URL

any idea would be great Graham