Need Software for chromecast single windows

Hi everyone,

i work as a teacher. In lessons i need to chromecast from my laptop to a digital board. I know that u can do it with Chromium, but Chromium will only cast the whole desktop. I only want to show the pupils xournal for drawings and writings, so that i can use on the other side my laptop to do something other, that the pupils should not see.
This is impossible with Chromium. I know that i can use VLC with screen recording. But everytime i use this and i use “playback” - “Renderer” i can connect but i get an black screen. Nothing is seen on the digital board. This works only with cable.
So is there any other possible (easy way) to realize that? Other teachers use screen sharing with apple, they dont have problems to do that.
I cant think, that linux cant do that in another easy way.