From reviewing this post and the information available from the Salesforce Marketing website, I would recommend that you pivot over to using their REST API as the SOAP API is only going to support classic tools. If you want to continue to use the SOAP API then this link Salesforce Developers - authenticate-soap-api outlines how to authenticate. It starts with a call to the Get Token Endpoint (Salesforce Developers - get-access-token) using the payload below
Then you will extract the accessToken payload
and inject it into a SOAP Header
I've attached a sample policy to help get you started. It will probably need some tuning.