In Windows 8, the automatic definition update work only IF you TURN ON the Windows Update and configure it to install update AUTOMATICALLY!
If you need to turn off the windows automatic update but still want Windows Defender to update its definition, it would be trouble.
According to some references from the internet, you can use the Task Scheduler to help you.
Use administrator right to open a command prompt and run the following:
schtasks /create /tn “My Definition Update” /sc DAILY /MO 1 /st 04:00 /ru SYSTEM /RL HIGHEST /tr “‘C:\Program Files\Windows Defender\MpCmdRun.exe’ -SignatureUpdate -MMPC”