How do I remotely add a Macbook to Apple Business Manager?

You can't. There is no way to add a device remotely. If you purchased a device via retail, then you can use Apple Configurator on your iPhone to inject the device into ABM. But this can only be done when the device is at Setup Assistant and your phone is in proximity. If the device is already setup, you must erase all content and setting to get it to run Setup Assistant.

Remember, the point of ABM is to enforce chain of custody. The first link in the chain is the legal contract you signed with Apple when you setup ABM, linking your legal business entity into a trust with Apple. This trust extends to the way you purchase the hardware. If you purchase it as your business identity (direct from Apple or from a DEP-aware reseller), then Apple can link the hardware to your ABM. If you walk into an Apple Store and buy retail, there is no way to establish a link in the chain.

If the device is already setup and in use, you can manually enroll the unit by installing the Company Portal app and performing a user-initiated enrollment.

Hope this helps,

Reid

The only way to remotely add the device is to purchase it from Apple or an Authorized Reseller who supports Apple Business Manager. They can add it for you at purchase. Some can add it after, but the user will have to run a sudo terminal command or wipe their device to benefit from it.