We can achieve the requirement with the following code :

Firstly, write the Java code or create a java function to establish the URL connection and read the response and make the attach stream out of that.

java.net.URL url = null;
java.net.URLConnection connection = null;

try {

    // Establish URL connection and hit the URL

    String URL = tools.getParamValue("URL");
    url = new java.net.URL(URL);
    connection = url.openConnection();
    connection.setDoOutput(true);

    java.io.BufferedReader in = new java.io.BufferedReader(
        new java.io.InputStreamReader(
            connection.getInputStream()));

    String decodedString;
    String htmloutput = "";

    while ((decodedString = in .readLine()) != null) {


        // Customise according to requirement

        // If the CSS URL path is not fully defined in the page source, then replace with full path to view the webpage exactly it looks online .. 
        //Do the same for all images as per the business requirements


        if (decodedString.contains(".css") && !(decodedString.contains("http"))) {
            int StartIndex = decodedString.indexOf("href");
            int EndIndex = decodedString.indexOf(".css");
            String replaceString = decodedString.substring(StartIndex + 6, EndIndex + 4);
            boolean StartsWithflag = replaceString.startsWith("/");
            if (StartsWithflag) {
                decodedString = decodedString.replace(replaceString, URL + replaceString);
            } else {
                decodedString = decodedString.replace(replaceString, URL + "/" + replaceString);
            }
        }

        // End of customization

        htmloutput = htmloutput + decodedString;
    }

    byte[] buffer = htmloutput.getBytes();

    strFileData = Base64Util.encodeToString(buffer);

} catch (java.io.IOException e) {} finally {
    try {

    } catch (Exception w) {}
}

Once we have the AttachStream, then create the WorkAttachFile and Link the attachment with the work object. That's it. The html page will be attached to the work page.

Note: This code works for the repository storage type.

Modify the code according to the requirements.