You can't do that at the moment. It'd be a good addition though - I've just added in issue for it: https://github.com/espruino/Espruino/issues/859
However, I guess you're expecting to use this on the ESP8266? I'm afraid the watchdog isn't implemented for that.
@Gordon started
Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
You can't do that at the moment. It'd be a good addition though - I've just added in issue for it: https://github.com/espruino/Espruino/issues/859
However, I guess you're expecting to use this on the ESP8266? I'm afraid the watchdog isn't implemented for that.