<field id="active" translate="label" sortOrder="220" showInDefault="1" showInWebsite="1" showInStore="0">
    <label>active buy</label>
    <frontend_model>[Vendor][Ext]BlockSystemConfigFormFieldActive</frontend_model>
    <backend_model>MagentoConfigModelConfigBackendSerializedArraySerialized</backend_model>
</field>

....

protected function _prepareToRender() {
    $this->addColumn('col_1', ['label' => __('Column 1'),  'renderer' => false]);
    $this->addColumn('col_2', ['label' => __('Column 2'),  'renderer' => false]);

    $this->_addAfter = false;
    $this->_addButtonLabel = __('Add');
        $this->_addAfter = false;
        $this->_addButtonLabel = __('Add');
}

...