I was stuck too, then I found a nice little guide on youtube. Look up "How to unlock the bootloader on the HTC Desire 510 (or any htc phone)" (sorry, I can't post links yet). The guy's a bit more specific than the dev site is. The only difference between the phone used in that guide and the 626s is that you don't need to go into fastboot and can stay in download mode.
Also, make sure to enable OEM unlocking in the developer options. I didn't know I had to do this and it slipped me up at first. Follow the steps and you should be fine :good:
---------- Post added at 07:38 AM ---------- Previous post was at 07:33 AM ----------
Anyone have any luck rooting yet? Kingo, Towelroot, and Kingroot all failed for me, which is strange because the towelroot site says "should support all phones < jun 3 2014." Guess it's wrong