Hi 梓珺 张,
DaRT is licensed through Volume Licensing with SA only, if your company has the required license, logging in to https://admin.microsoft.com with a user with the correct role assigned, it should be available under "Billing" / "Your products" / "Volume License" , and look for Microsoft Desktop Optimization Pack (MDOP).
(It is not available as a public download)
Hope this helps,