You technically can enter anything in that field that's an email address format, but we would advise using a real address. That way, you'll receive any replies from people who write back to you.
Additionally, it's a part of our terms of service that you'll clearly identify the origin of your email:
You also agree to ensure that email messages sent through your Emma account are truthful and accurately identify the source of the message. In all emails, you agree to use subject lines that are in no way false or misleading as to the identity of the sending organization or the nature of the content contained in the email. You also agree to include your organization's physical address in all campaigns, as required by the Can-Spam Act of 2003, 15 U.S.C. 7701, et seq.