This is the example of the user registration and file post via mongodb stitch webhook: (So in this case you can just use fetch to execute proper webhook)

exports = function(payload) {
    const request = EJSON.parse(payload.body.text());
    const http = context.services.get("<app-title>");
    const owner = context.values.get("<username-of-the-app-owner>");
    const apiKey = context.values.get("<api-key>");

    return http.post({
        url: "stitch.mongodb.com/api/admin/v3.0/auth/providers/…",
        body: JSON.stringify({
            username: owner,
            apiKey: apiKey
        })
    }).then(response => EJSON.parse(response.body.text()).access_token).then(accessToken => {
        return http.post({
            url: "stitch.mongodb.com/api/admin/v3.0/groups/<grou…",
            headers: {
                Authorization: ["Bearer " + accessToken]
            },
            body: JSON.stringify({
                email: request.useremail,
                password: request.userpass
            })
        });
    });
};

This is the one that uses aws s3 service:

exports = function(payload) {
    //base64EncodedImage, bucket, fileName, fileType
    const body = EJSON.parse(payload.body.text());
    // Convert the base64 encoded image string to a BSON Binary object
    const binaryImageData = BSON.Binary.fromBase64(body.picture, 0);
    // Instantiate an S3 service client
    const s3Service = context.services.get('<aws-s3-service-title>').s3('<location>');
    // Put the object to S3
    return s3Service.PutObject({
        'Bucket': '<aws-bucket-title>',
        'Key': body.fileName,
        'ContentType': body.fileType,
        'Body': binaryImageData
    })
        .then(putObjectOutput => {
            // console.log(putObjectOutput);
            // putObjectOutput: {
            //   ETag: <string>, // The object's S3 entity tag
            // }
            return putObjectOutput;
        })
        .catch(console.error);
    // return body;
};

What errors are you seeing?

Typically, you would use any server-side code in getInitialProps (you might check for request to make sure it only runs server-side). Browser SDK should probably be used on componentDidMount (or any regular React lifecycle method).