The prependToFTQuery prepends itself to the full ft query string. So I think you should end your prependToFTQuery with ' AND '. The searchvalue is added directly to it, but it need to be a valid searchstring.Take a look in the source code, you'll find this in de custom control:
var q = compositeData.prependToFTQuery + context.getUrl().getParameter("searchValue");
Where 'q' is the full query string.