Hello @jennyle,
It looks like the feature hasn't been enabled yet for your demo account. I highly recommend that you create a support case from your production account and inform the support expert that the custom email domain feature needs to be enabled for your demo account.
The workaround is to submit a case without logging into your account by using the “I can’t access my account” option at the bottom of the page, then click “Other issues” and provide complete details of the issue in the Case description.
Or you may click this link: Get Support.
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Best regards,
Ma. Cassandra | Docusign Community Moderator
"Select as Best" below if you find the answer a valid solution to your issue