For any Rotax n00bs… Since the water pump is internal to the engine, turning the rear axle won’t do the trick on the old FR125. I just fill my rad up very slowly, making sure the stream of water gets down into the guts of the radiator without splashing. The idea is to let the water level rise slowly and displace the air so there’s little if any to burp. You can run the engine briefly on the stand (or long enough to get warm if you have a thermostat), then pop the cap off to see if it’s burped, but I’ve still never had to top it off after the initial fill.
Goes without saying to drain the water out following the session… Many more sub-freezing nights ahead. 😥