The first time you do it, you’ll have no idea. If you track heart rate, you might have a sense of what threshold feels like. Maybe not. Others suggested a ramp test - it is good for beginners. There’s a 2x 8minute FTP test as well. Try either or both of those over the span of a couple of weeks and you’ll have a better sense of both your target FTP and a 20 minute pacing strategy.