@Hassib, Thanks for posting in Q&A.
From your description, I know you want to disable BitLocker and enter to change boot sequence at startup on your Surface RT8.1 device.
Here are links with useful information you can refer to.
https://thequickadvisor.com/how-do-i-disable-bitlocker-on-surface-rt/
https://xdaforums.com/t/cannot-enter-uefi-on-surface-rt.4491805/
Non-official, just for reference.
Hope it will help.
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