[promptfile] Use `_form_hidden_inputs`

youtube_dl/extractor/promptfile.py

@@ -35,10 +35,7 @@ class PromptFileIE(InfoExtractor):
             raise ExtractorError('Video %s does not exist' % video_id,
                                  expected=True)
 
-        fields = dict(re.findall(r'''(?x)type="hidden"\s+
+        fields = self._form_hidden_inputs(webpage)
-            name="(.+?)"\s+
-            value="(.*?)"
-            ''', webpage))
         post = compat_urllib_parse.urlencode(fields)
         req = compat_urllib_request.Request(url, post)
         req.add_header('Content-type', 'application/x-www-form-urlencoded')