https://xdaforums.com/t/the-quest-for-a-bootloader-unlock-partial-guide-t-mobile-variant-may-work-for-metropcs-metro-variant.4313525/

We can uninstall the app automatically as I said.

adb shell pm uninstall --user 0 com.qualcomm.qti.uim

Then the device has to reboot into bootloader mode. We receive the unlock code, imei and serial number in the following commands, which should be parsed and temporarily saved (I think these commands are universal, but I only tested on my Moto G5).

fastboot oem get_unlock_code

fastboot getvar imei

fastboot getvar serialno

After that we should output all of these cleanly, so that the user can create an account here.

https://www.oneplus.com/unlock_token

We cant speed up the unlock process, which takes ~7d and we cant expect the user to keep this app opened for that time, so we might have to implement the same procedure as the bootloader unlock/custom recovery checkboxes to skip forward. The progress of the unlock code generation can be checked here and you can download the unlock_code.bin once finished.

https://www.oneplus.com/unlock_token?type=2

Lastly we can flash cust-unlock and start normal unlocking and flashing procedure after that. We should ask for the file path of unlock_code.bin to automate that.

fastboot flash cust-unlock $somePath/unlock_code.bin

Using this procedure the user has to input the required info to create the account (which will be described in the app), download the unlock_code.bin, select the file and then just go through the normal process.
I think you noticed that I didnt contribute actual code yet, since Im not into python, but Im planning to learn. For now I need some help to implement that ig...