AWS Amplify: How to add SSR with API call on dynamic page?

I used AWS Amplify to host a NUXTJS site. SSR working fine for fixed page after creating a build with "npm run generate".
But when I set the build command "npm run build" to build server side rendering. I don't know where to add 'npm start' in Amplify, so that the Nuxt site run.
Can anyone help?