Finally got an input form. :-)

lostplaces/lostplaces_app/templates/voucher-verify.html

@@ -8,8 +8,8 @@
 <h2>Voucher-Überprüfung</h2>
 <form method="post">
   {% csrf_token %}
-  {{ form.as_p }}
-  <button type="submit">Login</button>
+  {{ voucher_form.as_p }}
+  <button type="submit">Überprüfen</button>
 </form>
 
 {% endblock maincontent %}