How To Use the Random Function in JavaFX

For a school assignment we have to develop a board game. Right now we're working on the dice system. I'm a beginner at JavaFX so I'm kind of having a hard time figuring out the 'Random'-function. I found a piece of code but it doesn't seem to work (see image)

We haven't been thought to work with 'public class' so I'd love a solution for this layout (stage, content).

So in short: how to make a random number generator for the numbers 1 - 6?

Thanks in advance :)