How to have a window on all workspaces?

You have to right-click on the title bar, when the window is not maximized and select "Always on visible workspace"

Pressing alt+space will display a menu where you will find "Always on visible workspace".

If it's a chrome window, then you need to right click on the title bar, choose "use system title bar and and borders". Then you get a new title bar, right click on it and choose - always on visible workplace.