In order to use these endpoints, you must first be authenticated - you will need an access token.
If you are looking for an easy way to authenticate, you can generate one by using the authorization code grant type.
Using your access token, you will need to add this header to all https://account-public-service-prod.ol.epicgames.com requests:
Authorization:bearer (your access token)