Sorry to hear you are getting a missing_app_token when trying to link you Hitbox account. Just wanted to check are you trying to link this through www.freedom.tm/dashboard/hitbox . If you are still getting some difficulty in linking this you could always contact the Support team via www.support.tm and they can look into this for you
If you can, capture a HAR file and send it to [email protected] along with your Freedom! Dashboard ID found at https://freedom.tm/dashboard/profile to have an engineer look into this. I've personally had the issue solved like that.