Answer
Azure Shared Key authentication in IFS Connect's REST Transport Connector is built for the Azure Blob Storage endpoint specifically — it isn't the same mechanism Service Bus expects (Service Bus needs its own SAS-token generation, not a raw shared key passed the Blob Storage way), so it won't authenticate against a Service Bus endpoint no matter how the key is formatted. Set up an Azure AD app registration and use OAuth2 client-credentials authentication against the Service Bus endpoint instead.