After a limited preview earlier in March, Microsoft Defender Advanced Threat Protection (ATP) is now available in public preview for MacOS devices.
To get this preview, you need to have first turn the preview feature in your Security Center portal (see https://docs.microsoft.com/en-us/windows/security/threat-protection/microsoft-defender-atp/preview)
All the documentation is available here https://docs.microsoft.com/en-us/windows/security/threat-protection/windows-defender-antivirus/microsoft-defender-atp-mac
You can on-board Mac devices from the Settings configuration blade and then accessing the Machine Management\Onboarding section
