How to generate a content-length - node.js

Question

I'm using the node module form-data to send multipart/forms to a server, but i'm not sure how to calculate the content-length correctly for the request. The form-data github documentation seems a bit hand-wavy about it and the server i'm sending to requires the content-length header to be included.
const FormData = require('form-data');
const fs = require('fs');
const https = require('https');
class Platform {
_postKey(key, serviceName) {
const options = this._getOptions();
const keyName = (key.search(/(PUBLIC)/) !== -1) ? 'publicKey' : 'privateKey';
const extension = (keyName == 'publicKey') ? 'pub' : '';
const filePath = (extension.length > 0) ? __dirname + '/data/'+serviceName+'.'+extension : __dirname + '/data/'+serviceName;
const file = fs.createWriteStream(filePath, {flags: 'wx'});
file.write(key);
file.end();
const form = new FormData();
form.append('file', fs.createReadStream(filePath));
form.append('Name', keyName);
form.append('MimeMajor', 'application');
form.append('MimeMinor', 'x-pem-file');
form.append('Extension', extension);
form.append('FileClass', 'MFS::File');
options.headers = form.getHeaders();
options.headers.Authorization = 'Basic ' + Buffer.from(this.FILESTORE_USERNAME + ':' + this.FILESTORE_PASSWORD).toString('base64');
// options.headers['Content-Length'] = key.length;
console.log(options);
return new Promise((resolve, reject) => {
let post = https.request(options, (res) => {
let data = '';
if (res.statusCode < 200 || res.statusCode > 299) {
reject(new Error('File Storage API returned a status code outside of acceptable range: ' + res.statusCode));
} else {
res.setEncoding('utf8');
res.on('data', (chunk) => {
data += chunk;
});
res.on('error', (err) => {
reject(err);
});
res.on('end', () => {
console.log(data);
if (data) {
resolve(JSON.parse(data));
} else {
resolve();
}
});
}
});
post.on('error', reject);
form.pipe(post);
});
}
}