JSFiddle - React, Tailwind, and code Playground

HTML

<textarea></textarea>
<textarea></textarea>
<textarea></textarea>
<a href="#" id="b" >Add Text</a>

JavaScript

var previouslyFocused = null;

let textareas = document.querySelectorAll("textarea");
textareas.forEach(t=>{
	t.addEventListener("focus",(e)=>{
		window.previouslyFocused = e.target;
  })
})

document.getElementById("b").addEventListener("click", function(e) {
  e.preventDefault();
  var text='test',
  		activeTextArea,
  		elements = document.querySelectorAll('textarea');
      
      if(window.previouslyFocused == null){
      	activeTextArea = elements[0];
      }else{
      	activeTextArea = window.previouslyFocused;
      }
      
  var strPos = activeTextArea.selectionStart,
     	front = (activeTextArea.value).substring(0, strPos),
      back = (activeTextArea.value).substring(strPos, activeTextArea.value.length);
      activeTextArea.value = front + text + back;
      strPos = strPos + text.length;
      activeTextArea.selectionStart = strPos;
      activeTextArea.selectionEnd = strPos;
      activeTextArea.focus();
});