set js to "document.forms[0].elements[0].value"
tell app "Safari" to set JOBDescription to (do javascript js in document 1)
Where “0” would mean “the first element in the first form”, so you can adjust it to match your needs, depending on the HTML source.
If you have the name, you can also use it:
set js to "document.forms[0].txtDescription.value"
--> or
set js to "document.forms[0].elements['txtDescription'].value"
You could even substitute “forms[0]” with the form name, if any. Eg: