The process is more or less similar to that of htc one m7. So for those who already unlocked sprint htc one m7 this should be familiar.
Requirements:
1. This tutorial is independent of Android version whether it may be 4.4.2 or 4.4.3. Works on both stock and sense based custom ROM's. (Not tested on AOKP ROM's.). Works on both s-off and s-on devices.
2. You need to be rooted and bootloader needs to be unlocked. Here are the links that would show you how to do it.
http://theunlockr.com/2014/04/19/root-htc-one-m8-all-in-one-toolkit-method-video/ '> https://www.dropbox.com/s/74r6h5q6ft50z6j/SIM_UNLOCK.zip?dl=0 '> https://www.dropbox.com/s/74r6h5q6ft50z6j/SIM_UNLOCK.zip?dl=0
4. Download and install Terminal from play store.
5. Switch off the handset and boot to fastest.(By holding volume down and power button together ).
6. Once you are on fastboot screen navigate to recovery using volume buttons and press power button to boot to recovery.
7. One you are in recovery click on install (I'm presuming you installed twrp recovery, if not then do it according to that recovery). Now locate SIM_UNLOCK. zip and flash it. Reboot system after successful flash.
8. On Reboot:Open the Terminal Emulator from the app drawer
9. Type - su
10. Hit enter on your keyboard
11. After you see the # type this- am start com.redbend.vdmc/com.htc.omadm.test.TestMainActivity
12. Hit the enter key
13. Next you should see a menu listing 3 options as follows:
GUI
API
ROAMING OPTIONS
14. Select API
15. Once the API menu comes up, scroll down until you see - sim lock
16. Tap- sim lock. Wait until the gray highlight goes away and turns black again
17. Reboot and you are done.
18. If you see you that you have no network then dial *#*#4636#*#*. A menu will pop up. Select phone information then scroll down until you see 'select preferred network type'.
Change it to WCDMA preferred and that is. Now your network should pop-up.
This is permanent unlock and you can update your Android version in future.
Thank you.