Subdomain or root domain?
A sending subdomain keeps root MX free for inbound.
Send from a subdomain such as mail.example.com. Your root domain's MX records stay with Google, Microsoft, or whoever hosts your mailboxes, and your sending setup stays separate from your everyday mail.
How it works
The sending domain is the host you verify. Once mail.example.com verifies, you can send from any address on it, such as hello@mail.example.com or receipts@mail.example.com. There is nothing else to create.
If you also want to receive mail, the default receiving host is inbound.mail.example.com. You can pick another host under the sending domain, or the sending domain itself, as long as that host has no MX records.
Sending from the root domain
Verifying example.com works too. You then send from addresses like hello@example.com. Receiving on the root is allowed only when the root has no MX records, which is rare for a domain that already has mailboxes.