You don’t need to add anything, but if the water content of the sugar syrup is well controlled, it will naturally become hard. Sometimes you will see paintings painted with sugar syrup on the street, which means the water content is controlled just right. If there is too much water, it will still be soft after cooling. You need to

Just do a few more experiments and you will know. My experience is that when boiling sugar, the foam changes from large to small, and when the sugar becomes dilute from light to dark, lower the heat. Put a few drops in a bowl of cold water. It is estimated to have cooled down. You can judge by taking it out and pinching it.

As long as you can’t pinch it! Once you get familiar with it, you can grasp it just by looking at it! I wish you success!