You can assign shortcuts via:

 View -> Toolbars -> Customise:

Here's an example.
Select a container:
 Main Form -> Main menu (menu bar)
Now select an action in the middle pane of the pop-up screen:
 Folder -> Mark all message as read
Now set the focus on the input box for the shortcuts, that's the
little bar at the right hand bottom that says 'None' or maybe 'Geen'
in your case.
Now press a shortcut. You'll see the key combo is mentioned in the
input box and that the 'Assign' button lights up.
Press the assign button, and the key combo will move up to the
assigned shortcuts for the selected action.

You can only assign shortcuts to actions in a container and said
action shortcut is only available when the container is relevant. So
shortcuts assigned to the pop-up window of the message list won't work
when you're editing a message or when you've got focus on the account
tree pane.

Less awkward than you think, when you realize that you can use roughly the same procedure to add items to menus, toolbars and pop-up screens.