i am trying to use a custom app with Admin API and i have an access token that starts with "shpat_" if sent in headers with "X-Shopify-Access-Token" it works perfectly!
Then try using the http_request node, add the headers in the http request node (depending on node-red version) Or set them prior to the node using a change or function node, using msg.headers["X-Shopify-Access-Token"] the request body goes in msg.payload.