Change value of an INPUT TEXT checking a checkbox

Why is this not working? It is supposed to work this way: if the checkbox is checked, it should take the content of a variable txt and write it to the input with id of show.

function myFunction() {
  if (document.getElementById('myRadio').checked) {
    var txt = "Done";
    document.getElementById("show").innerHTML = 'txt';
  }
}

<p>If the radio checkbox is checked the text from the variable text is written into input line with the ID of show</p>
        
<input type="checkbox" onclick="myFunction()" id="myRadio">

<br>
<br>

<input type="text" class="form-control" id="show">