From http://docs.oasis-open.org/mqtt/mqtt/v5.0/mqtt-v5.0.html

This specification requests IANA to modify the registration of the WebSocket MQTT sub-protocol under the “WebSocket Subprotocol Name” registry with the following data:

Figure 6.6‑1 - IANA WebSocket Identifier
Subprotocol Identifier mqtt
Subprotocol Common Name mqtt
Subprotocol Definition http://docs.oasis-open.org/mqtt/mqtt/v5.0/os/mqtt-v5.0-os.html