Don't know about these cars but most cars now have the plastic expansion tanks ( white bottle) which is where you top up. Can't see how you can top up the radiator itself unless that has a cap.
The bottle has a min and max mark or just max mark thats the level it should be check with car on FLAT ground for true level mark. Don't undo cap quick if you've been running engine it will spout scaulding water as these are under pressure.
Turn a little you should hear a low hiss then stop, allow 3 mins for unheard pressure to escape then place thick cloth over cap and take cap off.
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