On jitsi I see the chat, hand, but not the desktop sharing button, is there something special to do in order to get this feature ?
For chrome you need an extension https://github.com/jitsi/jidesha.
This will change in the near future, with new chrome versions, but is still not the case.
Actually I didn 't mean on the client side, but on the server side.
If I connect to meet.ji.si I have the button to share screen, on my side I don’t with my jitsi instance.
Yes, you need to configure the extension id in config.js. You get the id by creating an extension for your deployment and upload it to chrome webstore.