Figured it out.
Create the API credentials in Fluent / Wordpress
In n8n create credentials with basic auth
In the hTTP node use basic auth and choose the correct credential.
Fluent docs say its needs header auth but hitting url https://yoursite.comapp/wp-json/fluent-crm/v2/subscribers works with basic auth - it returns json data with contacts data
I needed to stop wordfence from blocking application passwords in wordpress.