UPDATE, see here: https://github.com/amcolash/flowkey-scraper - Noteflight auto transcribe

flowkey.js

// Find the sheet music images on the page
const elements = document.getElementsByClassName('split-image');

// Check that there are actually images on the page
if (elements.length === 0) {
  console.error('No images found');
} else {
  // If images were found, extract the base url from the 1st one
  const imageUrl = elements[0].src;
  const imageIdMatch = /\/sheets\/([\w\d]+)\//;
  const baseUrl = 'https://flowkeycdn.com/sheets/';

  // Construct the final url to use
  const matched = imageUrl.match(imageIdMatch)[1];
  const url = `${baseUrl}${matched}/300/`;

  // Log the url to the console
  console.log(url);

  // This last line may fail on some browsers, but you can always manually copy from the log statement above.
  copy(url);
}

noteflight.sh

#!/bin/bash

# Note: To run on large pieces of music, you will need to edit /etc/ImageMagick-6/policy.xml
# to allow stiching a very large image together in ImageMagick. I had these extra lines in my file:
# <policy domain="resource" name="width" value="64KP"/>
# <policy domain="resource" name="height" value="64KP"/>

# Check that only one argument (the base url was passed)
if [ "$#" -ne 1 ]; then
  echo "Usage: flowkey [base url]"
  exit 1
fi

# Check that imagemagick is installed
if ! [ -x "$(command -v convert)" ]; then
  echo 'Error: imagemagick is not installed.' >&2
  exit 1
fi

# Check that audiveris is available (you will need to check out the repo and follow install instructions)
if ! [ -d ~/Downloads/audiveris ]; then
  echo 'Error: audiveris not installed. Please clone the repository to ~/Downloads/audiveris'
  exit 1
fi

# Move to /tmp
pushd /tmp

# Set up temp folder
rm -rf flowkey_tmp
mkdir flowkey_tmp
pushd flowkey_tmp

# Continually try to grab images with wget until this fails. At that point, we know we have the entire song.
i=0
while true; do
  URL=$1$i.png
  wget -nv $URL
  if [ $? -ne 0 ]; then
    break
  fi

  i=$((i+1))
done

# Stitch all images together and fill the background of the output with white
convert $(ls *.png | sort -V) +append stitched.png
convert stitched.png stitched.png -background white -layers merge output.png

popd

# Copy the result to the desktop
cp flowkey_tmp/output.png ~/Desktop/flowkey_music.png

# Run audiveris (maunally you will need to run the OMR)
pushd ~/Downloads/audiveris
./gradlew run
popd

# If you have chrome, open noteflight in a new tab to complete things
if ! [ -x "$(command -v google-chrome)" ]; then
  google-chrome https://www.noteflight.com/home
fi