You must have a paid subscription in order to receive free backups. Free backups are being added the quota of your subscription.
In your BackHub account, you will find a unique invitation link you can use to invite new users to BackHub. For each new user, using your invitation link to sign up for BackHub for the first time, you will receive 10 free repository backups if the new user purchases a paid plan and the account stays active for at least 1 month after the 14 day free trial.
When signing up for BackHub using an invitation link, you must have cookies activated when opening the invitation link and start installation from there in order to be able to receive 10 free private repository backups. Free private repository backups can not be applied to your account in retrospect.