How to send Base64 from front-end to API

I am attempting to send a fax with the Vitelity API. I have an API on EC2 that I am calling from the front-end of my React Native app, and that in turn calls the Vitelity API:

const Base64String = async (req, res) => {
  let body = req.body;
  await postFax(body);
  return res.status(200).send({
    statusCode: 200,
    data: {
      ...body,
    },
  });
};

const postFax = async ({
  login,
  pass,
  faxnum,
  faxsrc,
  recname,
  file1,
  data1,
}) => {
  const VITELITY_URL = `https://api.vitelity.net/fax.php?login=${login}&pass=${pass}&cmd=sendfax&faxnum=${faxnum}&faxsrc=${faxsrc}&recname=${recname}&file1=${file1}&data1=${data1}`;

  await fetch(VITELITY_URL, {
    method: "POST",
  })
    .then((res) => res.text())
    .then((result) => {
      console.log(result);
    })
    .catch((err) => console.log(err));
};

data1 is Base64 encoded and works with a test image I have that is ~24k characters, however some of the images I’m taking have been upwards of 1.2m characters.

The exact issue I’m getting is 414 Request-URI Too Large.

This is the front-end:

const sendB64 = b64 => {
    let formdata = new FormData();
    formdata.append('login', testLogin);
    formdata.append('pass', testPass);
    formdata.append('cmd', 'sendfax');
    formdata.append('faxnum', destinationNumber);
    formdata.append('faxsrc', testSenderNumber);
    formdata.append('recname', 'Test');
    formdata.append('file1', 'testfax.jpg');
    formdata.append('data1', b64);

    let requestOptions = {
      method: 'POST',
      headers: {
        'Content-Type': 'multipart/form-data',
      },
      body: formdata,
      redirect: 'follow',
    };

    fetch(API_URL, requestOptions)
      .then(response => response.json())
      .then(result => {
        console.log(result);
        showMessage({message: `status: ${result.statusCode}`});
      })
      .catch(error => console.log('error', error));
  };