TYPO3 selectTree, choose everything not in relation as Startingpoint

Without any startingPoints the selectTree just renders this empty selectTree:

But I want it to load every Item it has in its relationship
basicly setting the startingPoints dynamically to every item not sorted in
it should look like this:

My current TCA:

'config' => [
    'type' => 'select',
    'renderType' => 'selectTree',
    'multiple' => true,
    'MM_hasUidField' => true,
    'foreign_table' => 'tx_mysite_domain_model_item',
    'allowed' => 'tx_mysite_domain_model_item',
    'MM' => 'tx_mysite_domain_model_item_mm',
    'treeConfig' => [
        //'startingPoints' => '???'
        'childrenField' => 'children',
        'appearance' => [
            'expandAll' => true,
            'showHeader' => true,
        ],
    ],
]