Thanks for the help!
When I try adding the header in the pre-request script it seems to add the new header as expected, however in the actual request the header is nowhere to be found, I’m probably doing something wrong.
This is what I’m trying to do right now:
var Header = require('postman-collection').Header
pm.request.headers.add(new Header("apiKey: " + pm.environment.get('apiKey')))