JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" id="userInput" placeholder="First, type something here.">

<button type="button" onclick="copyPaste"();>Then, click here to copy the last character<br>of the above input into the next input.</button>

<input type="text" id="input2" value="Some text...">

CSS

input {
  width: 255px;
}
  
button {
  display: block;
  margin: 20px 0;
  text-align: left;
}

JavaScript

function copyPaste () {
  var i1 = document.getElementById('userInput');
  var i2 = document.getElementById('input2');
  var c = i1.value.substr(lol.length - 1);
  c.select();
  document.execCommand('copy');
  i2.focus();
  document.execCommand('paste');
  i1.focus();
}