How to generate signed URL of file in google cloud storage using only OAuth 2.0 bearer token and bucket name? (PHP code)

I have tried the code below and it works fine. However, it would be ideal if I could just use the OAuth 2.0 access token and bucket name to get the signed URL.

I have tried many things but still nowhere.

function getSignedURL($file, $expiryMinutes = 30)
{

    $privateKeyFileContent = '{
     // service account generated key
    }';

    $storage = new StorageClient([
        'keyFile' => json_decode($privateKeyFileContent, true)
    ]);
    $bucket = $storage->bucket("as-portal");
    $object = $bucket->object($file);
    $url = $object->signedUrl(
    # This URL is valid for 15 minutes
        new \DateTime("$expiryMinutes min"),
        [
            'version' => 'v4',
        ]
    );
}

Any help would be greatly appreciated!