I might be cheeky, but you are looking for an Alarm OUT interface which serves as a relay INPUT (not an output).
You have to look for (on Dahua NVR) something like this:
As you can see, there might also be Alarm IN plugs, they can, for example, trigger a PTZ/tour/record action on a cam.
There are even IPC who have an Alarm OUT interface.
Good luck!
CC