Yeah that's what I've done, using the function node beforehand. So no way of using context variables directly? I thought this was a feature in some nodes.
msg.token = global.get("TOKEN");
return msg;
Then add msg.token to header field in http request node.
This is extra node and extra code to extract the value, instead of simply using it directly in the http request node. Hoped there was a way to do this with less code (less code is usually better code) 
Ideally, the http request node would have an option like this:
source:
Variables in edit fields - General - Node-RED Forum (nodered.org)
