Keyboard Maestro is a powerful automation tool for Mac that can do all sorts of things. Over on MacStories, they show how to use it so you can write text in any app, then automatically send it elsewhere with a quick keyboard shortcut.
The idea here is to replicate the now-defunct QuickCursor, which made it so you could press a keyboard shortcut in any text window (like your browser), open up your favourite text editor, then automatically send that text back to the original text form. This is great for things like email or writing online when you might prefer a more powerful text editor. With the Keyboard Maestro macro below, you can replicate this feature. Open a text window, tap the keyboard shortcut, then you're taken to the text editor of your choice. The text will automatically get updated in the original text field. You can customise it to work with any app of your choice.