I followed this tutorial: https://rapiddg.com/blog/creating-ajax-callback-user-reference-fields

to create a button to fill some field in a form with the values of the current user.

Here the cose I added to my hoo_form_alter:

$form['user_replace'] = array(
    '#title' => t("User Information"),
    '#prefix' => '<div id="datore_node_form_group_cs">',
    //name of the node form (already existing, if you're modifying an existing node form)
    '#suffix' =>'</div>',

    $form['user_replace']['user_copy'] = array(
      '#type' => 'button',
      '#value' => 'Sono io il datore!',
      '#ajax' => array(
        'event'=>'click',
        'callback' =>'user_replace_ajax_callback',
        'wrapper' => 'datore_node_form_group_cs',
      ),
    ),
  );

 $form['user_replace']['field_cognome'] = $form['field_cognome'];
  unset($form['field_cognome']);

  $user = user_load($user->uid);
  dpm($user);
  dpm($user->field_cognome['und'][0]['value']);
  dpm($form);
  dpm($form_state);

  if (!empty($form_state['values']['field_cognome']))
  {
    unset($form_state['input']['field_cognome']['und'][0]['value']);
    $form_state['input']['field_cognome']['und'][0]['value'] = $user->field_cognome['und'][0]['value'];
    return $form;
  }

  function user_replace_ajax_callback($form, &$form_state)
  {
    ajax_command_alert("hello"); // doesn't work
    return $form['user_replace'];
  }

but if I click the button nothing happen. Any advise?

I followed this tutorial: https://rapiddg.com/blog/creating-ajax-callback-user-reference-fields

to create a button to fill some field in a form with the values of the current user.

Here the cose I added to my hoo_form_alter:

if ($form_id == 'datore_node_form')
{
global $user;
  $form['title']['#type'] = 'hidden';
  $form['title']['#value'] = 'test';

$form['user_replace'] = array(
    '#title' => t("User Information"),
    '#prefix' => '<div id="datore_node_form_group_cs">',
    //name of the node form (already existing, if you're modifying an existing node form)
    '#suffix' =>'</div>',

    $form['user_replace']['user_copy'] = array(
      '#type' => 'button',
      '#value' => 'Sono io il datore!',
      '#ajax' => array(
        'event'=>'click',
        'callback' =>'user_replace_ajax_callback',
        'wrapper' => 'datore_node_form_group_cs',
      ),
    ),
  );

 $form['user_replace']['field_cognome'] = $form['field_cognome'];
  unset($form['field_cognome']);

  $user = user_load($user->uid);
  dpm($user);
  dpm($user->field_cognome['und'][0]['value']);
  dpm($form);
  dpm($form_state);

  if (!empty($form_state['values']['field_cognome']))
  {
    unset($form_state['input']['field_cognome']['und'][0]['value']);
    $form_state['input']['field_cognome']['und'][0]['value'] = $user->field_cognome['und'][0]['value'];
    return $form;
  }

  function user_replace_ajax_callback($form, &$form_state)
  {
    ajax_command_alert("hello"); // doesn't work
    return $form['user_replace'];
  }

but if I click the button nothing happen. Any advise?