how can I set the default value for the field "priority" in the actions form?
Bernard, you can do this by developing a little extension and using the buildForm hook to set the default value of the priority. In the buildForm hook you can do something like this:

if ($form-&gt;_action == CRM_Core_Action::ADD) {
  try {
    $normaal = (int) civicrm_api3('OptionValue', 'getvalue', [
       'return' =&gt; "value",
       'option_group_id' =&gt; "priority",
       'name' =&gt; "Normal",
    ]);
    $defaults['priority_id'] = $normaal;
    $form-&gt;setDefaults($defaults);
  }
  catch (CiviCRM_API3_Exception $ex) {
    Civi::log()-&gt;warning(E::ts('Onverwachte fout: geen prioriteit met naam Normal gevonden in ') . __METHOD__ . E::ts(', foutmelding van API OptionValue getvalue: ') . $ex-&gt;getMessage());


}
}

More information on the buildForm can be found here: https://docs.civicrm.org/dev/en/latest/hooks/hook_civicrm_buildForm/

Does this make sense?