Thank for the answer.
Since ILMerge is not supported, is there another way to reference "Microsoft.Azure.ServiceBus" namespace inside a Plugin?
Using "Microsoft.Azure.ServiceBus" namespace is possible to access to the "UserProperties" of the message and use Azure Service Bus Rules to redirect the message to the appropriate subscription.
AFAIK, the only possibility to send a message to ASB from plugin is by "IServiceEndpointNotificationService" but this service does not allow to add "UserProperties" disabling the possibility to use the powerful feature of the ASB Rules.
Can you suggest me some resource on either include some Microsoft namespace inside a plugin or on customize message headers using "IServiceEndpointNotificationService"?