If you own a custom domain, you can connect it to Proton to create custom email addresses. You will then be able to send messages from addresses other than @protonmail.com.
Custom domains also allow you to add more users to your Proton account by creating an organization and assigning them custom email addresses. Please note that you can’t create an organization without connecting a custom domain and that the users you add to your organization can only use custom email addresses, and not @protonmail.com or @pm.me addresses.
To use this feature, you need a paid Proton plan and your own domain. If you don’t have one yet, you can buy one through domain registrars like namecheap.com. Free domains can be quickly acquired and typically cost around $10 per year.
To connect your custom domain to Proton, you need to access the domain’s DNS records and change them to point to ProtonMail. That’s why you can’t create custom email addresses from domains you don’t own or control, such as .edu or .gov domains.