How do I setup the watch to swim 8*50m freestyle where I kick off every 50 sec (pause depending on my speed) without programming a workout in advance?
You don't... you either set up a workout in advance, or you just go and swim it. For the latter, set up the rest screen on your watch to include the "interval time" after you complete each 50, then go when it the interval time hits :50. There is no pre-programming outside of the workout feature.
For 50s especially, I would not use autorest because by the time the watch figures out that you've stopped you dont have much time to look at your watch. So I do manual stop/start.