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="allid" value="EHM39972"/>'
but use html/body[1]/form[1]/input[1]/@value can't get the result 'EHM39972'
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 your value like you do for other usual extractions.