How to parse an S3 JSON file into environment variables for Serverless.ts

What is the syntax to parse an S3 .json file into individual environment variables in the Serverless.ts configuration?

I’ve tried the following without success to get the environment variable MY_SECRET set to the value of MY_SECRET in the bucket’s JSON.

functions: {
    hello: {
      environment: {
        MY_SECRET: "${s3:my-config/services/test/support-portal-api.json}"
      },

…this deploys but sets MY_SECRET to the entire contents of the JSON file

functions: {
    hello: {
      environment: {
        MY_SECRET: JSON.parse("${s3:my-config/services/test/support-portal-api.json}").MY_SECRET
      },

…this fails to deploy with :

SyntaxError: Unexpected token $ in JSON at position 0
      at JSON.parse (<anonymous>)

What is the correct syntax? The documentation isn’t very clear. Please note this is a Typescript (Serverless.ts) file.