I remember when young, my mother showed me how to fill it. First, add the hot/ boiling water, cap it then shake vigorously. The bottle will expand, carefully unscrew the plug a little to release pressure. That's easy.
I have fond memories of Hills. From the rotary clothes hoist we had growing up. Also as I entered the electronics industry, they were leaders with their TV antennas and accessories. Proven the best. The staff I got to know was very well, and helpful. Sad times.
Here in WA, our Bunnings sausage sizzles are served in a proper bun, as they should be. Yes, onion if you wish and a variety of sauces that you apply. (y)