Under "My Account", "Account Settings", and "My Preferences", you will notice a field called "Domain". By default it will show a random string of numbers, but you are encouraged to change that to your company name. That way when your client opens an invoice, contract, quote or questionnaire, it will show your company name in the URL so it looks very professional.
If you try to edit any other field in the "My Preferences" section, you will be prompted to change your domain.
Keyword Search: Domain, Numbers, Subdomain