Test page
jQuery(document).ready(function() {
function getUrlVar(key){
var result = new RegExp(key + "=([^&]*)", "i").exec(window.location.search);
return result && result[1] || "";
}
jQuery("input[name=inf_field_FirstName]").val(decodeURIComponent(getUrlVar('FirstName')));
jQuery("input[name='inf_field_LastName']").val(decodeURIComponent(getUrlVar('LastName')));
jQuery("input[name='inf_field_Company']").val(decodeURIComponent(getUrlVar('Company')));
jQuery("input[name='inf_field_EmailAddress3']").val(decodeURIComponent(getUrlVar('Email')));
});