I have discussed about unlocking bootloader for Hipad Pro for flashing Custom ROMs on another post. The usual command lines for Android platform tools are as follow: (Please apply these in bootloader mode)
fastboot flashing unlock
fastboot oem unlock
fastboot oem unlocking
Follow this link for usual method of unlocking Android device with no required official unlock tool.
IF there is the requirement for the official unlock tool, this link shows the example of the process for unlocking bootloader with the official unlock tool.
If none of the above command lines work for your device, please check out this link for hidden command lines that may help you unlock bootloader for Hipad Pro.
This thread is specifically for unlocking bootloader for Hipad Pro. So in the future users of Hipad Pro will google search this easily in the future.
Hope we will find out the successful technics for unlocking bootloader of this device.
Remark: My Hipad Pro is unlocked by default already from the factory or from the seller.