89c0777d_HiveMQ_1_4_1___User_Guide_Table_3__Static_properties_Description
[Property Name] websockets.subProtocol [Default Value] mqttv3.1 [Description] 
The subprotocol used for websockets. When using Paho.js this should be mqttv3.1
HiveMQ 1.4.1 - User Guide
Description