The topic that the server should publish the WillMessage to in the event of an ungraceful disconnection.
The Will feature of MQTT allows a client to specify to the server a WillMessage to publish (as well as a WillTopic to publish it on) in the event that the server considers the client lost or ungracefully disconnected.
An "ungraceful disconnection" is any disconnection other than one triggered by calling Disconnect. If the client enters sleep mode, the server will not publish the Will. However, if the client is lost as a result of exceeding the sleep duration without sending a message, the server will publish the Will.
In addition to the WillTopic and WillMessage properties, the WillQOS setting may be used to specify the Will message's QoS level, and the WillRetain setting to set the Will message's Retain flag. Refer to those settings for more information.
If WillTopic is set to empty string (default) when Connect is called, the class will not send a Will to the server.
This property is not available at design time.