Sample Auth0 action to execute on a specific API and Application for adding a custom claim.

m2m_action.js

/** @type {CredentialsExchangeAction} */
module.exports = async (event, context) => {
  // This action works for an API with the API audience https://example.com/api along with an application with the 
  // client id 5drbxrf5qMc1KTFNEw6Wjrbw319pOiyW
  if (event.audience === "https://example.com/api" && event.client.id === "5drbxrf5qMc1KTFNEw6Wjrbw319pOiyW") {
    return {
      customClaims: {
        "https://www.customnamespace.com/some_key": "some value"
      }
    };
    } else {
      return {};
  } 
};