Input text in sweetAlert

Use Sweetalert2. There are numerous examples for input prompts to be found here, all of them using modern async/await constructs. If you want it the old way, try this:

Swal.fire({
    title: "An input!",
    text: "Write something interesting:",
    input: 'text',
    showCancelButton: true        
}).then((result) => {
    if (result.value) {
        console.log("Result: " + result.value);
    }
});

swal({
  title: "An input!",
  text: "Write something interesting:",
  type: "input",
  showCancelButton: true,
  closeOnConfirm: false,
  animation: "slide-from-top",
  inputPlaceholder: "Write something"
},
function(inputValue){
  if (inputValue === null) return false;
  
  if (inputValue === "") {
    swal.showInputError("You need to write something!");
    return false
  }
  
  swal("Nice!", "You wrote: " + inputValue, "success");
});

JSFiddle

Give the input the autofocus tag.

text: '<br /><form method="post" id="taxcode-update" name="taxcodeUpdate">'
    + '<input id="admin-tax-code" autofocus minlength="3" class="form-control wedding-input-text wizard-input-pad" type="text" name="taxCode" placeholder="Codice fiscale">'
    + '</form>',