function THEME_form_search_block_form_alter(&$form, &$form_state, $form_id) {
  $form['#attributes']['class'][] = 'form-inline ';

}

/*
* Implement hook_form_alter()
*/
function custom_form_search_block_form_alter(&$form, DrupalCoreFormFormStateInterface $form_state, $form_id) {
    $form['#attributes']['class'][] = 'test';
}

function THEMENAME_preprocess_form(&$vars) {
  $vars['attributes']['class'][] = 'form-inline ';
}

function YOURTHEMENAME_form_search_block_form_alter(&$form, &$form_state, $form_id) {
$form['#attributes']['class'] = myclass; // Change the class name.