Convert an existing SPA to SSR

[DrakoPOD]

I'm trying to convert my App to SSR, I'm following the example of the doc, but I have problem with this line:

"scripts": {
  "build:client": "vue-cli-service build --dest dist/client",
  "build:server": "SSR=1 vue-cli-service build --dest dist/server",
  "build": "npm run build:client && npm run build:server",
}

It's says that SSR is not defined, how I can define this environment variable?