How to make REST API call to create contact by just using API key from the server?

I have an app where I want to create each user who signs up on the platform as a contact on the ConstantContact. I am looking to create contact from the backend NodeJS, without OAuth flow. But I haven't found any document for the REST API to create a contact with just API key and Secret key. Please advise or point to the document that helps me address the problem.

Thanks