When you sign up for an Aha! trial account, you will have the opportunity to create a custom domain name in the following format: https://xxx.aha.io. This is the domain name that you will give to your users to access your account.
If the domain name is already taken, you can pick an alternate name for the time being. Don’t worry — you can always change it later if you decide to convert to a paid account at the end of your trial.