I want to display the query string (if it's not empty) as the value of an input field, something like this `<input name="campaign" type="hidden" value="ABC Campaign {% if (QueryString.GetValue("utm_source") != null) { return "| QueryString.GetValue("utm_source")"} %}" />`

but couldn't make it work due to all the double quotes. I've tried escaping the quotes with back slash but didn't work. Could you help?
(The above code is placed inside the **Form > Layout**)