WS_SECRET NOT SET

[hatemmhb]

I have been set the configuration in app.json as below:
[ { "name" : "geth3", "script" : "app.js", "log_date_format" : "YYYY-MM-DD HH:mm Z", "merge_logs" : false, "watch" : false, "max_restarts" : 10, "exec_interpreter" : "node", "exec_mode" : "fork_mode", "env": { "NODE_ENV" : "production", "RPC_HOST" : "localhost", "RPC_PORT" : "8551", "LISTENING_PORT" : "30301", "INSTANCE_NAME" : "", "CONTACT_DETAILS" : "", "WS_SERVER" : "https://localhost:8545", "WS_SECRET" : "255", "VERBOSITY" : 2 } } ]
Then run frontend using the command npm start
but I always have this error
"WS_SECRET NOT SET!!!"
How do i set the WS_SECRET and where?