Ko Bagan,

Dell does not have any generic key codes for Windows 8.1. You could click the link below to request  backup media.

Dell Backup Media Request US/Canada ONLY.

They key is in the BIOS.

You can get the key from the BIOS after Windows is installed, using RWEverything (ACPI, MSDM).

If you are using media to install that asks for the key during the install, you can do one of two things:

1) Use a KMS key to get past the prompt:
https://technet.microsoft.com/en-us/library/jj612867.aspx

2) You can boot to a Linux CD and get it using the following command:
sudo hd /sys/firmware/acpi/tables/MSDM

See here to download Windows 8.1 with Update 1 Media:

http://dellwindowsreinstallationguide.com/download-microsoft-windows-and-office/download-microsoft-windows/download-windows-8-1-retail-and-oem-iso/

Make sure you accept the correct edition which will automatically input your UEFI BIOS SLP key.