@JsonProperty("join")
        public String getPrimaryConnection() {
            return primaryConnection;
        }
Returns the opaque ID of the connection being joined (shared), if
any. If specified, any provided protocol is ignored. This value is
exposed via the "join" property within JSON.

@return
    The opaque ID of the connection being joined (shared), if any.
extensions/guacamole-auth-json/src/main/java/org/apache/guacamole/auth/json/user/UserData.java