This is what I wanted to achieve:
First, you submit an authentication request to https://auth.listrak.com/OAuth2/Token with your client id and client secret. If that is successful, you receive a bearer token to use in further API calls. Once authenticated, we need to process a REST API call to https://api.listrak.com/email/v1 with encoded data (ie. email address, etc). I don’t know what should be flow, currently I am running my script in the “addtional checkout”, but it throws an “Cors” error as attached in the screenshot.
The same post I found:
https://community.shopify.com/c/shopify-apis-and-sdks/can-shopify-handle-server-side-api-rest-calls-listrak-and-oauth/m-p/554313#M36680