When you want to map a menu item to a short cut you can do so in the 'edit - keyboard shortcuts' menu. However, that will only work when you want to target only that specific menu item.
But when you want that menu item to be part of a whole set of steps recorded in an action, you are going to need the 'Insert Menu item...' command from the Actions panel. Once you have recorded that action you can assign it to a function key along with the shift or command/ctrl key.
The problem here is you are limited to the function keys.
To combine all of these steps (menu item - action - any keyboard shortcut) you are going to need a script. Which then triggers the action, which then does select that menu item. And all this can be assigned to any key. That process we will cover in a follow up to this tutorial.