<?php

$columns = [
    'external_identifier' => [
        'label' => 'External identifier (my.typo3.org)',
        'exclude' => 1,
        'config' => [
            'type' => 'input',
            'readOnly' => true,
        ],
    ],
];

\TYPO3\CMS\Core\Utility\ExtensionManagementUtility::addTCAcolumns(
    'tx_t3omembership_domain_model_member',
    $columns
);
\TYPO3\CMS\Core\Utility\ExtensionManagementUtility::addToAllTCAtypes(
    'tx_t3omembership_domain_model_member',
    'external_identifier',
    '',
    'after:hidden'
);