And it adds the value "10". The problem is that it adds "10" to the last value. 
I want it to add 10 to just the value I want. 

Example: www.mypage.com?test=123&hello=321
How would I do it if I just wanted the "10" on the test variable (but only if variable "test" exist) so it would turn out to:
www.mypage.com?test=12310&hello=321

Apprecite any help, thanks 

// Daniel

My script: 

$('a').attr('href', function (index, value) {

  // If there is a query string appy this to links.

  if (window.location.search) {

    // If the link allready contains a question mark add the query

    // string with an ampersand instead.

    if (value.indexOf('?') > -1) {

      return value + '&' + window.location.search.substr(1);

    }

    return value + window.location.search + 10;

  }

});