Whenever, you try to access the advanced recovery options of your Surface tablet with Windows RT, Windows may require you to enter the BitLocker recovery key provided. Without providing the key, you can’t use the advanced options like Refresh, Reset, Automatic Repair etc on your Surface tablet.
Microsoft provides the BitLocker recovery key to every person using Windows 8 with a Microsoft account and the key is unique for every device and account but, you just have to access the key which is allotted automatically to you.
Accessing the BitLocker recovery key requires the following conditions:
You must be using your Windows 8 with a Microsoft account
You must use a trusted PC to access the recovery key
You can access the BitLocker recovery key by going to this link from your trusted PC (you may need to sign-in to your Microsoft account).