Add field internal_reference to invoice addresses

src/pretix/base/models/orders.py

@@ -881,6 +881,11 @@ class InvoiceAddress(models.Model):
     vat_id = models.CharField(max_length=255, blank=True, verbose_name=_('VAT ID'),
                               help_text=_('Only for business customers within the EU.'))
     vat_id_validated = models.BooleanField(default=False)
+    internal_reference = models.TextField(
+        verbose_name=_('Internal reference'),
+        help_text=_('This reference will be printed on your invoice for your convenience.'),
+        blank=True
+    )
 
 
 def cachedticket_name(instance, filename: str) -> str: