Webform populate text component with encoded url values
You're on the right track. It looks like you have a syntax problem with your URL param itself. If you're trying to get the value of ?this, remove the sumbitted and  [ (%5D) and ] (%5B). So your URL with the param will look like this:

http://example.com/abc?this=1


With that URL, [current-page:query:this] should return 1 (or TRUE).