On the NVR create a rule in Setup -> Event -> Event Rule Setup
Select Manual Trigger for the Event Trigger and User Coding as the Event Action
Enter the following string in the text field:
ptzcontrol.cgi?msubmenu=aux&action=control&Command=WiperOn
Then on the Live screen, on the camera view you will see a Manual Trigger icon, clicking on this will activate the camera wiper
