$resourceId = $modx->getOption('resource', $scriptProperties, $modx->resource->get('id'));
$tvName = $modx->getOption('tv', $scriptProperties, '');

// Get the TV
$tv = $modx->getObject('modTemplateVar',array('name'=>$tvName));

// Get the raw content of the TV
$rawValue = $tv->getValue($resourceId);

return $rawValue;
Source

$tvName = $modx->getOption('tv', $scriptProperties, '');

// Get the TV by name
$tv = $modx->getObject('modTemplateVar',array('name'=>$tvName));

// Get the ID of the TV
if ($tvName) {
    $id = $tv->get('id');

    return $id;
}
Source