BitLocker is a device encryption feature of Windows. If your device is asking you for your BitLocker recovery key, there is no "back door," there are no workarounds, and Microsoft support can't provide you with the missing key or create a new one for you. You will need that 48-digit key to unlock your device.
Here are the places you can check to locate the key:
In your Microsoft account: Sign in to your Microsoft account on another device to find your recovery key. This is the most likely place to find it.
Note: If the device was set up, or if BitLocker was turned on, by somebody else, the recovery key may be in that person’s Microsoft account.
On a printout you saved: Your recovery key may be on a printout that was saved when BitLocker was activated. Look where you keep important papers related to your computer.
On a USB flash drive: Plug the USB flash drive into your locked PC and follow the instructions. If you saved the key as a text file on the flash drive, use a different computer to read the text file.
In an Azure Active Directory account: If your device was ever signed in to an organization using a work or school email account, your recovery key may be stored in that organization's Azure AD account associated with your device. You may be able to access it directly or you may need to contact a system administrator to access your recovery key.
Held by your system administrator: If your device is connected to a domain (usually a work or school device), ask a system administrator for your recovery key.