-        fields = dict(re.findall(r'''(?x)type="hidden"\s+
-            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')
+        chash = self._search_regex(
+            r'val\("([^"]*)"\s*\+\s*\$\("#chash"\)', webpage, 'chash')
+        fields = self._hidden_inputs(webpage)
+        keys = list(fields.keys())
+        chash_key = keys[0] if len(keys) == 1 else next(
+            key for key in keys if key.startswith('cha'))
+        fields[chash_key] = chash + fields[chash_key]
+