How to replace default value of field?

I have a custom module.

This is form_alter function:

 $form['article'] = [
    '#type' => 'textfield',
    '#weight' => -1,
    '#title' => t('Search Product from Magento: '),
    '#autocomplete_route_name' => 'magento_drupal.autocomplete',
    '#ajax' => [
      'callback' => '_select_ajax_form',
      // 'wrapper' => 'my_unique_id',
    ],
  ];

In the _select_ajax_form function, how can I automatically fill in the data for the field 'price' and 'sku'?

price is Number (integer) and sku is Text (plain, long).

If you don't have a huge backend logic for the auto-fillins, you may do it with much more simple javascript in the "#attributes" value. Here's a sample of my code:

$form['laufzeit_in_monaten'] = [
      '#type' => 'number',
      '#title' => $this->t('Laufzeit in Monaten ('.$produkt->get('min_stk')->value.' - '.$produkt->get('max_stk')->value.')'),
      '#default_value' => floor($produkt->get('max_stk')->value/2.0),
      '#attributes' => array('onchange' =>
        'document.getElementById("vollpreis").innerHTML = (document.getElementById("edit-laufzeit-in-monaten").value * document.getElementById("pprc").value).toFixed(2)'),
    ];

But this is only frontend. You obviously need to validate the input later in your backend.