JSFiddle - React, Tailwind, and code Playground

HTML

<textarea id="txt" rows="15" cols="70">There is some text here.</textarea>
<input type="button" id="btn" value="OK" />

JavaScript

var btn = document.getElementById("btn"),
	txt = document.getElementById("txt");

if (btn && txt) {
	btn.addEventListener("click", function () {
		var caretPos = document.getElementById("txt").selectionStart,
			textAreaTxt = txt.value,
			txtToAdd = "stuff";
    
    txt.value = textAreaTxt.substring(0, caretPos) 
    	+ txtToAdd 
      + textAreaTxt.substring(caretPos);
    
    txt.focus();
    txt.selectionStart = caretPos + txtToAdd.length;
    txt.selectionEnd = caretPos + txtToAdd.length;
    
	});
}