Either that, e.g. in the shell:
NOTIFY_WHAT=HOST
NOTIFY_HOSTSTATE=CRIT
NOTIFY_HOSTADDRESS=192.4.18.312
...
./custom-snow.py
Or fake a check result in the checkmk GUI to trigger a notification:
Sure. There will be a line that says
executing /omd/sites/.../local/share/check_mk/notifications/custom-snow.py