Add custom submit handler to a form

function mymodule_form_alter(&$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id)
{
       if ($form_id == 'form_id_to_alter'){
               $form['actions']['submit']['#submit'][] = '_my_submit_function'
       }
}


function _my_submit_function(array &$form,  \Drupal\Core\Form\FormStateInterface $form_state)
{
//add needed actions
}