How can I make the text I select in the window write itself?

If you want text to “write itself,” you can use a typing animation effect. In After Effects, apply the Typewriter preset; on websites, use libraries like Typed.js; and in PowerPoint/Word, apply an animation that reveals text “by letter.” If you mean auto-inserting selected text, that can be done with automation tools like macros or AutoHotkey scripts.
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