Hi
Sadly you can't buy it alone, you need the Windows E5 subscription for your desktop. It's something I talked to that team PM, as it's hard to integrate that product for selling it.
A resume of the requierement;
Licensing requirements
Microsoft Defender for Endpoint requires one of the following Microsoft volume licensing offers:
Windows 10 Enterprise E5
Windows 10 Education A5
Microsoft 365 E5 (M365 E5) which includes Windows 10 Enterprise E5
Microsoft 365 A5 (M365 A5)
Microsoft 365 E5 Security
Microsoft 365 A5 Security
Microsoft Defender for Endpoint
See more there; https://learn.microsoft.com/en-us/microsoft-365/security/defender-endpoint/minimum-requirements?view=o365-worldwide
Thanks
Philippe