I have written an AppleScript application and when running, it appears in the Dock, however I would like it to have it in the menu bar, at the top of the screen, instead. How can I achieve this? Is there some code I need to add to it so it can appear in the menu bar?
For a single script, it is no possible to create a unique icon that's sits near the clock such as the wifi icon for example. There may be unique AppleScript plugins that enable this, but I'm not aware of any.
If you need one icon from which you can run a bunch of different scripts, there are tools like FastScripts that enable this. OS X also enables application-specific scripts to show up in a special icon on the menu bar that only shows for that app. Not sure that still works in El Capitan, as I haven't used this functionality for a while.
I run a tool called Keyboard Maestro which lets me trigger scripts in a variety of different ways, one of them being the menu bar icon.