<?php
namespace UVBestellungPlugin;

class Buttons
{
    public function beforePushButtons(
        MagentoBackendBlockWidgetButtonToolbarInterceptor $subject,
        MagentoFrameworkViewElementAbstractBlock $context,
        MagentoBackendBlockWidgetButtonButtonList $buttonList
    ) {

        $this->_request = $context->getRequest();
        if($this->_request->getFullActionName() == 'sales_order_view'){
            $buttonList->add(
                'Bestellschein',
                ['label' => __('Bestellschein'), 'onclick' => 'alert("test");', 'class' => 'reset'],
                -1
            );
        }
    }
}
?>

<?php
namespace UVBestellungControllerAdminhtmlOrder;
class Custom  extends MagentoSalesControllerAdminhtmlOrder
{
    public function execute()
    {
        // my logic here
    }
}
?>