Note: This is an administrator-only feature.
When you have opened Organization Settings, stay on the first tab "Details" and click on "Add Domain" in the "Allowed Email Domains" area.
A pop-up will then appear for you to add the domain along with an email address for us to send a verification. Once it has been verified, it is added to your allowed email domain list along with the date added.
To remove an added email domain, simply locate the domain, click on the three dots and select "Remove"