What is your purpose - - what would that menu item do?
Would it be added to the standard menu items (and used with the Finder), or would it be added when launching an app?
An app would need to access menu items for a variety of functions, so the easiest method would be including your menu items as part of designing an app, and you should consider registering as a developer with Apple.
Keep in mind that developers have restrictions for location and use of items in SOME of the menus, such as the left end Apple menu., and third party apps are supposed to follow Apple's guidelines for use and appearance.
Do you have a background in programming, including app design and use of the various graphic elements that would be used in any app?
(Choose the IDE you want to use --- sign up with Apple's Developer site )
I want a simple menuItem to store frequently used text such as my e-mail address.So every time I need to enter one of these items I can quickly go to the standard apple menubar click on my menuitem, select the text I want, and have it filled in the required field. I have some knowledge of programming but use AppleScript most of the time.
I thought there would be an existing app to do this but I could not find one.
My first thought would be that a menu item might get lost, if you have another app open, then decide you want to go to your "storage" menu item. (Some apps might not leave a space for your custom menu item, and just ignore it, unless you can figure out how to make it system-wide, so it is accessible at all times). Apple developers usually have limits on how they can bypass the standard Apple menu layout, and "custom" menu items would probably be ignored entirely by some apps (and your custom menu item might not even be visible when another app is active)
Sounds to me like it would be easier to have an icon in the dock for your saved text file, where it would always be one-click away.
I use much-the-same thing for rarely used internet logins.
And, you might want to try out your Automator app, as that can make simple things like you want, easy to use.
There's other apps on your Mac that do similar tasks: Notes, or Stickies, or Reminders, etc.