On your way to becoming a shortcut ninja you may have chosen to hide the menu bar in your Signal Desktop preferences.
To view the menu again, open Signal Desktop and press Alt+F to bring up the File menu and access Signal's preferences so you can re-enable the menu bar.
Here are more shortcuts:
| Mac | Windows/Linux | |||
| View all shortcuts | ⌘ / | Ctrl / | ||
| View Signal Desktop preferences | ⌘ , | Ctrl , | ||
| View the menu again | Alt F |