I need to type a character similar that looks like space, in Windows I would've used Alt+255. But I don't know how to do this Ubuntu... Can I somehow use Alt codes in Ubuntu, or use another shortcut?
Either Ctrl+Shift+U00A0 or AltGr+Space: it's called a non-breaking space.
See this thread on Ubuntu Forums for more info.
Press the Compose key (default Shift+AltGr although you can change it using
gnome-tweak-tool), then press Space twice.
Shift+AltGr Space Space