silverstripe - add dataobject button to cms / modeladmin

gistfile1.php

class BillingAgreement extends DataObject
{

  public function getCMSActions() {
    $fields = parent::getCMSActions();

    $button = new FormAction('doMarkProcessed', 'Mark Processed');
    //$button->addExtraClass('ss-ui-action-constructive');
    $fields->push($button);

    return $fields;
  }
  
  // ...
}

class BillingAgreementAdmin extends ModelAdmin {
  public static $managed_models = array(
    'BillingAgreement'	=> array('title' => 'Billing Agreements'),
    'BillingAgreementDefaultLineItem'	=> array('title' => 'Default Line Items'),
    'BillingAgreementEmailTemplate'	=> array('title' => 'Email Templates')
  );
  
  public function getEditForm($id = null, $fields = null) {
    $form = parent::getEditForm($id, $fields);
    $gridField = $form->Fields()->fieldByName($this->sanitiseClassName($this->modelClass));
  
    $config = new GridFieldConfig_RecordEditor();
    
    if($this->modelClass == 'BillingAgreement') {
        $config->addComponent($export = new GridFieldExportButton());
        $config->getComponentByType('GridFieldDetailForm')->setItemRequestClass('BillingAgreementForm_ItemRequest');
    }
    
    // ...
    
    $gridField->setConfig($config);
    return $form;
  }
}
  
class BillingAgreementForm_ItemRequest extends GridFieldDetailForm_ItemRequest {

  private static $allowed_actions = array(
    'edit',
    'view',
    'ItemEditForm'
  );

  public function ItemEditForm(){
    $form = parent::ItemEditForm();
    
    $actions = $this->record->getCMSActions();
    if($actions->exists()) {
        $form->Actions()->merge($actions);
    }

    return $form;
  }

  public function doMarkProcessed($data, $form) {
    $this->record->MarkProcessed();
    // ...
  }
}