How to send (encode) emojis (or any foreign language text) in the Subject of Gmail API?

I'm using the Gmail API in REACT NATIVE to send mails and using "js-base64" lib to encode the MIME message before sending out through the api. Everything works fine but the emoji's in the Subject part isn't decoded the right way

(Edited) This is the gmail send mail api call

const sendEmail = async (message: ISendMessage) => {
  const data = JSON.stringify({
    raw: getMessageMime(message),
    threadId: message.threadId,
  });
  const response = await G_POST(data, SEND_EMAIL_URL, 'application/json');
  return response.data;
};

const getMessageMime = (message) => {
  let mail = [
    'Content-Type: multipart/alternative; boundary="foo_bar_baz"\r\n',
    'MIME-Version: 1.0\r\n',
    'To:' + message.to + '\r\n',
    'From:' + message.from + '\r\n',
    'Cc: ' + message.cc + '\r\n',
    'Bcc: ' + message.bcc + '\r\n',
    'References:' + message.inReplyTo + '\r\n',
    'In-Reply-To:' + message.inReplyTo + '\r\n',
    'Subject: ' + message.subject + '\r\n\r\n',
    '--foo_bar_baz\r\n',
    'Content-Type: text/plain; charset="UTF-8"\r\n',
    'MIME-Version: 1.0\r\n',
    'Content-Transfer-Encoding: 7bit\r\n\r\n',
    message.message + '\n\r\n',
    '--foo_bar_baz--',
  ].join('');

  mail = btoa(mail);
  return mail;
};