Hello all!
While not new to Linux, I am new to dealing with the boot process and configurations.

To avoid the XY problem I'll just detail what I've researched and discovered and hope someone more knowledgeable can figure out what's the best path to pursue.
I'd like a bootloader to switch between Zorin and Windows 10. However, my Windows install came from the manufacturer with Bitlocker setup. I installed Zorin with Secure Boot enabled, but when I select the Grub entry for Windows I get a prompt for the Bitlocker recovery key.

Next attempt was to add Zorin to the Windows Boot Manager, but easyBCD doesn't seem to support EFI systems.

Then I tried to disable/re-enable TPM as per superuser.com/questions/1278841. This fails for mysterious reasons though:
Some sort of OEM lock possibly? Or maybe I can't have BitLocker enabled while I change settings? A push for Windows 11? No idea.

Finally I saw that rEFInd has a 'Firmware Boot' option that works by setting the BootNext efi variable, but I wasn't sure how to install a whole new bootloader. So I went to see if grub has the same feature and found a rejected patch from 2013, and found https://github.com/datasone/setup_var.efi, but I've got no idea how you'd even begin to use it with grub.

Let me know if there's any additional information that could help. Zorin and Windows 10 are both fully up to date and everything was done to the defaults. Windows and Zorin are on different disks, but both are internally mounted in the laptop if that makes a difference.

[QUOTE]but easyBCD doesn't seem to support EFI systems.[QUOTE]

True, due to restrictions placed by microsoft. See the neosmart site at the link below for an explanation.

https://neosmart.net/wiki/easybcd/uefi/

Everything I have read indicates Bitlocker needs to be disabled. What happens when you enter the bitlocker key? Have you tried disabling it? You should be able to boot Zorin with Secure Boot on but try without it.

The version of windows 10 I have will not boot without TPM enabled while the Linux systems boot either way, enabled/disabled.

If Zorin and windows are UEFI, you should be able to select either from the BIOS boot options. If you want to boot both from Grub, disable Bitlocker to start and test if that works.

No, that is just Microsoft trying to get its hand in your wallet. I'm guessing you have Window Home - in which case you don't have access to the tools you need. They will suggest you upgrade to the Pro edition and you will - $169AU in my case; like hell.
I am just opening up a new laptop, and ran into this issue - most of the posts on the internet will not be applicable to you due to this limitation. I don't need bitlocker so turned it off (from settings, not the cmd prompt). I've got to iron out some other issues before I actually load Linux, but will likely be posting again.

From Microsoft documentation

After a bit of frustration, I did end up just entering the BitLocker key, which seems to have authorized Grub as the secure bootloading process. I need to re-enter it if I switch back to the Windows bootloader or significantly change grub, but that shouldn't matter. Thanks for the helpful suggestions!