JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text">
<button>Juntar palavra</button>

JavaScript

var input = document.querySelector('input');
var caret = 0;
var texto = '_olá!_';
input.addEventListener('keyup', function() {
    caret = this.selectionStart;
});
var button = document.querySelector('button');
button.addEventListener('click', function() {
    var val = input.value;
	input.value = val.slice(0, caret) + texto + val.slice(caret);
});