Use external resource in Word add-in with non-hard coded bearer token or is it possible to use an environment variable during runtime?

Question

Using a Word add-in, I'd like to make an external resource the subject of a fetch call, then inserted into the task pane. I've gotten it to work but only if I hardcode the environment variable in the fetch request header. A fairly recent question in Stack Overflow suggests that using environment variables during an add-in runtime isn't feasible. The question is how would I go about accessing the external resource which requires a bearer token that isn't hard-coded? Relatedly, can't seem to get process.env.<ENV_VAR> to work within taskpane.js