Configuration/FlexForms/flexform.xml

<T3DataStructure>
	<sheets>
		<sDEF>
			<ROOT>
				<TCEforms>
					<sheetTitle>Blog Config</sheetTitle>
				</TCEforms>
				<type>array</type>
				<el>
					<switchableControllerActions>
						<TCEforms>
							<label>Type</label>
							<config>
								<type>select</type>
								<items type="array">
									<numIndex index="0" type="array">
										<numIndex index="0">Listenansicht</numIndex>
										<numIndex index="1">Controller->action;</numIndex>
									</numIndex>
									<numIndex index="10" type="array">
										<numIndex index="0">Einzelansicht</numIndex>
										<numIndex index="1">Controller->action;</numIndex>
									</numIndex>
								</items>
							</config>
						</TCEforms>
					</switchableControllerActions>
				</el>
			</ROOT>
		</sDEF>
	</sheets>
</T3DataStructure>

ext_tables.php

$pluginSignature = str_replace('_','',$_EXTKEY) . '_pi1' ;
$TCA['tt_content']['types']['list']['subtypes_addlist'][$pluginSignature] = 'pi_flexform';
\TYPO3\CMS\Core\Utility\ExtensionManagementUtility::addPiFlexFormValue($pluginSignature, 'FILE:EXT:' . $_EXTKEY . '/Configuration/FlexForms/flexform.xml');