The Javascript in this gist can be used to grab a parameter (in this example, called "hop") from the URL's query string and place that value into a hidden form field (in this example, with an id of #inf_field_FieldName.)
There are two assumptions in this example, so you should alter them for your own use case.
For example, if you are accessing this page at http://example.com/?hop=something, then this example would retrieve a value of "something". If your URL parameter is called something other than "hop", change it on Line 8 of the example script from
document.getElementById('inf_field_FieldName').value = getUrlParameter('hop');