The equivalent of pressing the start button in script is "/tool sniffer start", and of course, "/tool sniffer stop" for stopping.
See this manual page
for all packet sniffer related commands and settings.
EDIT: Wait, I'm sorry... You meant "Wireless sniffer", not "Packet sniffer". I got confused by the combined "Wireless Packet Sniffer"...
For that, see this portion of the Wireless manual page
for the settings, and as for the command... It's called "sniff", in that very menu ("/interface wireless sniffer"). It works pretty much the same as how "print" works, except that the list is filled gradually.