KDE

There used to be a thing in KDE where you could execute actions and macros based on window titles, but it's since been removed. I've had to recreate this behavior with a shell script, here's a modified version you can use if you use Xorg:

#!/bin/bash

xprop -spy -root _NET_ACTIVE_WINDOW | grep --line-buffered -o '0[xX][a-zA-Z0-9]\{7\}' |
while read -r id; do
    class="`xprop -id $id WM_CLASS 2> /dev/null | grep Viber`"
    if [ -n "$class" ]; then
        if xprop -id "$id" | grep -q 'WM_NAME(STRING) = "Viber Window Name"'; then
            echo "key alt+f4" | xdotool -
            # wait for the window to be closed
            xprop -spy -id $id > /dev/null 2>&1
            fi
    fi
done

The first Viber is is looking for the Viber in the application name, replace "Viber Window Name" with the actual window title you want to be closed. If you don't use Xorg, you'll have to find a replacement for both xprop and xdotool.