How to auto-submit a form using Drupal Form API?
This one is a pretty easy method by adding 'onChange' to the '#attributes' array:

/**
 * Custom form handler.
 */
function FORM_EXAMPLE($form, &amp;$form_state) {

  $form['complete_projects_radios'] = [
    '#type'          =&gt; 'radios',
    '#title'         =&gt; t('Project status'),
    '#default_value' =&gt; 0,
    '#options'       =&gt; [
      0 =&gt; t('Current'),
      1 =&gt; t('Complete'),
    ],
    // Define the onChange like so:
    '#attributes'    =&gt; [
      'onChange' =&gt; 'this.form.submit();',
    ],
  ];

  $form['complete_projects_submit'] = [
    '#type'       =&gt; 'submit',
    '#value'      =&gt; t('Apply'),
    // The submit itself is necessary, but now it can be hidden:
    '#attributes' =&gt; [
      'style' =&gt; ['display: none;'],
    ],
  ];

  return $form;
}