Try this to debug:
(it will always match initially, but then you get rid of the
OR 1=1
after things are working.
.
RuleDescription rd = new RuleDescription();
rd.Filter = new SqlFilter("CustomProperty = '1' OR 1=1");
Then you can tweak the syntax sugar as needed.
https://learn.microsoft.com/en-us/dotnet/api/microsoft.servicebus.messaging.sqlfilter?view=azure-dotnet
You should show YOUR code on how you create the subscription. Below is some code.. as an example:
NamespaceManager nsm = /* outside of scope of this question */
SubscriptionDescription currentSubscriptionDescription = new SubscriptionDescription("MyTopicPath", "MySubscriptionName");
currentSubscriptionDescription.AutoDeleteOnIdle = false;
SubscriptionDescription postCreateSd = nsm.CreateSubscription(currentSubscriptionDescription);
/* now tweak it */
MessagingFactory mf = /* outside of scope of this question */
SubscriptionClient subClient = mf.CreateSubscriptionClient("MyTopicPath", "MySubscriptionName");
RuleDescription rd = new RuleDescription();
rd.Filter = new SqlFilter("CustomProperty = '1' OR 1=1");
subClient.AddRule(rd);