Click on a folder/collection.
Go to the Pre-request Script
tab and add something like:
pm.request.headers.add({
key: 'X-API-KEY', value:
pm.globals.get('API_KEY')
});

Click on a folder/collection.
Go to the Pre-request Script
tab and add something like:
pm.request.headers.add({
key: 'X-API-KEY', value:
pm.globals.get('API_KEY')
});