You have to unlock it within Fastboot. The regular OnePlus 6T and McLaren version both use exactly the same UEFI bootloader, except for power management part which is of no concerns, what we are actually interested in is the boot security.
The only end difference in how it's unlocked is boiled down to the carriers. Some aren't so friendly to the customers like Verizon whose have nasty habit of denying people the rights to unlock whatever they bought from Verizon for stupid reasons. *cough* Customers' experience. *cough*
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