Thanks @danny-dainton, judging from your comment I assume that the solution is indeed a pre-request script then? It’s outside the scope of the interceptor?
I think I got it wrong actually, I should keep response.json() because the response returned was actually in JSON format. I confused that with my request body sorry, so I didn’t need response.text() in the end. It’s all sorted now, thanks for your help. The issue was actually with me setting the environment variables wrong in the end.