Looking at your screen shot, your issue might be due to the "qs" and "sig" parameter values that could be dynamics. The expected timestamp could be serialized inside that long...
when post a form data, it would show exception page, from exception page, we find it is the from data encode problem, but i try two method to change the encode, it is still failed.
use xpath expression can get the element, but can't get the attribute value of the element.
such as:
html/body[1]/form[1]/input[1]/
-can get '<input type="hidden" name...
With your current Xpath expression you should be able to isolate a portion of your text containing your value. Then use the "Starting with" with the "Ending with" fields to grab...