using Htc desire 820s plz fastboot

hey friends
i am using htc desire 820S. its working well and booting to the installed android version but when i tried to ge to the boot menu it appears
""NO Command",, i can't boot to fastboot mode and others options. but when i pressed the vol up button i get into "android System recovery"
i want to unlock the boot-loader form htcdev.com for this fastboot is necessary...