It appears that your question concerns Azure Communication Services - if so, refer to https://learn.microsoft.com/en-us/azure/communication-services/quickstarts/email/add-custom-verified-domains?pivots=platform-azp
If you want to implement custom domains in Microsoft 365, then follow https://learn.microsoft.com/en-us/microsoft-365/admin/email/change-email-address?view=o365-worldwide
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hth
Marcin