Need keyboard shortcut to open system preferences

Which system version do you have installed?

On macOS 10.14.6 Mojave I can open open System Preferences from the Finder using this setting in System Preferences > Keyboard > Shortcuts > App Shortcuts:

• Click + to add a shortcut for the Finder and enter "System Preferences..." and a shortcut key, for example ^S.
• Restart the Finder with >Force Quit.

Now you should be able to use the ^S key to open System Preferences, when the Finder is in the foreground. Try to add a key for any application, where you want to open System Preferences.

You should be able to define a shortcut for the menu item > System preferences. The  menu is shown in any application unless you hide it in the Full screen view.

See if this helps: System Preferences shortcut not working - Apple Community

I don't believe it can be done with the System/Keyboard/Shortcut preference pane. I was unable to get any to work for all apps.

However, there's an excellent 3rd party macro app, Keyboard Maestro, which will let you run any menu, open any file or app, mount EHDs, paste text, etc. I use it and have over 100 macros to speed up my workflow. I have a key combination of Option + 1 to open the system preferences from any app.

They offer a trial version.

Just follow léonie’s advice with one important change: instead of adding the shortcut to the Finder, do it for “All Applications”.

Works like a charm here.

There is, as always, an important point: be careful with choosing the shortcut. You do not want to use something that is already in use by some application.

In addition: there are already several shortcuts to open specific panes of System Preferences: alt-F1 or alt-F2 for Displays, alt-F5 for keyboard... I will let you guess/find the others.