Prob applying X-SMTPAPI header in sendgrid-php.php

My attempt at supplying an X-SMTPAPI header in a fashion that the sendgrid-php.php library will accept is failing. Email goes out OK to the address supplied in the ->addTo but I have not yet been able to apply the ->addHeader header method or the json so it's accepted and acted upon... as you can see, the attempted json array is far shorter than the 1000 char line length allowed.

Any help or suggestions?

Here is the code:

    $email = new \SendGrid\Mail\Mail(); 
    $email->setFrom("", "That EMail");
    $email->addTo("", "Example Recipient");  //
    $email->addHeader('X-SMTPAPI','{"to":["", \n" \n"]}');
    //$email->addTo("bubba@example.xo", "Fail Recipient");  //TESTED OK
    //$email->addTo("", "Unsub Recipient"); //TESTED OK
    $email->addContent("text/plain", $txt);
    $email->addContent("text/html", get_option('EmailHTML'));
    $sendgrid = new \SendGrid(get_option('SendgridAPIKey'));
    try {
        $response = $sendgrid->send($email);
        print '<p>Code: ' . var_export($response->statusCode(),true) . "\n<br />Headers: ";
        print_r(var_export($response->headers(),true));  // future: use var_export instead.
        print '</p><br /><p>Body: ' . var_export($response->body(),true) . "\n</p>";
    } catch (Exception $e) {
        echo 'Caught exception: '. $e->getMessage() ."\n";