Authenticate request with API key

Hey Guys!

I am working on an integration to use some of the API features in one of our application. The problem is that I cannot use the oAuth or the access_token way to authenticate my requests so I would like to use the legacy XML-RPC API. The authentication is done but when I try to query to create a new contact with the official nodejs sdk (https://github.com/infusionsoft/infusionsoft-sdk-nodejs) I get these errors: No method matching arguments: java.lang.String, java.lang.String, java.lang.Integer, java.lang.Integer, [Ljava.lang.Object;'

What could be the problem? Is the legacy XML-RPC solution still works? Do we have any workarounds perhaps? Or can I use the new REST API with a single API key like in the legacy service?

Regards,
Gábor