Fix seatingframe missing voucher (#5734)

src/pretix/presale/templates/pretixpresale/event/seatingplan.html

@@ -25,6 +25,7 @@
       action="{% eventurl request.event "presale:event.cart.add" cart_namespace=cart_namespace %}?next={{ cart_redirect|urlencode }}">
     {% csrf_token %}
     <input type="hidden" name="subevent" value="{{ subevent.id|default_if_none:"" }}"/>
+    <input type="hidden" name="_voucher_code" value="{{ voucher.code|default_if_none:"" }}">
     {% if event.has_subevents %}
         {% eventsignal event "pretix.presale.signals.render_seating_plan" request=request subevent=subevent voucher=voucher %}
     {% else %}