How can I give Ajax access to currently set fields on my form?
I have tried this on my local it works . So you can achieve the form values by using this . Please try this 

    $form['filter_field'] = [
      '#title' =&gt; $this-&gt;t('Filter'),
      '#type' =&gt; 'select',
      '#options' =&gt; ['drupal7' =&gt; 'Drupal7', 'drupal8' =&gt; 'Drupal8'],
      '#empty_option' =&gt; $this-&gt;t('-select'),
      '#ajax' =&gt; [
        'callback' =&gt; '::updateWidgetAjax',
        'wrapper' =&gt; 'default-value-ajax-wrapper',
      ],
    ];
    $form['widget_type'] = [
      '#title' =&gt; $this-&gt;t('Widget Type'),
      '#type' =&gt; 'select',
      '#options' =&gt; ['pdf' =&gt; 'Pdf', 'ppt' =&gt; 'PPT'],
      '#empty_option' =&gt; $this-&gt;t('-select'),
    ];

    $form['wrapper'] = [
      '#type' =&gt; 'container',
      '#attributes' =&gt; ['id' =&gt; 'default-value-ajax-wrapper'],
    ];

    return $form;


//Ajax callback 
  public function updateWidgetAjax(array &amp;$form, FormStateInterface $form_state) {
   $widget = $form_state-&gt;getValue('widget_type');
    print_r($widget); //Print the value of the field
  }