Do I need to show a flight for onward travel when applying for a SETV in Kuala Lumpur?

Hi, all. I plan on applying for a SETV in Kuala Lumpur. I read that you need to show onward travel out of Thailand. Does it have to be a flight? If it's a flight, can it be one way to the next country without a return? If it doesn't have to be a flight, what's the cheapest option? Just to add, I don't plan on actually using the flight or onward ticket out of Thailand because my SETV will hopefully change to a Non-B in the meantime. Thanks!