If the endpoint really is a direct link to the .xls file, you can try the following code to handle downloading:
public static boolean download(final File output, final String source) {
    try {
        if (!output.createNewFile()) {
            throw new RuntimeException("Could not create new file!");
        }
        URL url = new URL(source);
        HttpURLConnection connection = (HttpURLConnection) url.openConnection();
        // Comment in the code in the following line in case the endpoint redirects instead of it being a direct link
        // connection.setInstanceFollowRedirects(true);
        connection.setRequestProperty("AUTH-KEY-PROPERTY-NAME", "yourAuthKey");
        final ReadableByteChannel rbc = Channels.newChannel(connection.getInputStream());
        final FileOutputStream fos = new FileOutputStream(output);
        fos.getChannel().transferFrom(rbc, 0, 1 << 24);
        fos.close();
        return true;
    } catch (final Exception e) {
        e.printStackTrace();
    }
    return false;
}
All you should need to do is set the proper name for the auth token and fill it in.
Example usage:
download(new File("C:\\output.xls"), "http://www.website.com/endpoint");