I couldn’t find a plugin that lets you do this out-of-the-box. If you’re using a keyboard that’s made for the language, it will often have a direct way to add a special character. I found this resource that shows how you can set up macrons in Windows for Māori.
The “short” way to insert a special character varies by Operating System.
For example, on a Mac you can simply hold down the option key and press “A” and a small dialog will allow you to choose from different accents for the letter. On Windows, you can do CTRL+SHIFT+~ then a. On most Ubuntu-based Linux distributions, you can type CTRL+SHIFT+U then E3.
Another way you could accomplish this is with a custom keyboard shortcut program, like AutoHotKey for Windows.