I've recently had to ask Hikvision's technical team about this to advise a customer with a similar query. There seems to be very little online about One-Key Disarming compatibility among the different NVRs.
Hikvision have told me that the feature is unique to NVRs running the GUI V4, so only the I-Series at current. They did not comment on whether it will be available on K-Series NVRS when/if they release V4 for them.
From my understanding, you can only access the feature through an NVRs local GUI and not through browser/iVMS-4200. To turn one-key disarming on, you'll need to go to:
System > Event > Normal Event > Alarm Input
Set up the desired alarm input, here's our
guide on how to set up alarms on Hikvision NVRs.
Once done, check the
One-Key Disarm box, as shown below, and then
Save.
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