Shortcut keys are for a single keystroke into a menu item. For example , "Alt-X" is a common shortcut to File-Exit. If you are only using the single letter after the menu has dropped down, then you just add a modifier to the text of the menu item.
exitToolStripMenuItem.Text="E&xit";
That also puts an underline under the "x", indicating to the user that is the key to press.