It works much like whiteboard.fi did/does. From your desktop you click the link, and then whiteboard and then join the board from your iPad.
I think this video would be the most useful https://www.youtube.com/watch?v=4V6JPM7uR4g (syncing whiteboard and companion device [iPad])
But this link goes through common questions https://www.mathsuniverse.com/whiteboard/#:~:text=Open%20the%20website%20on%20your,%F0%9F%93%B7%20to%20share%20a%20photo.