First you need to create the Icon for the ActionBar Follow this tutorial with the function as the following,

ClipboardManager clipboard = (ClipboardManager) getSystemService(CLIPBOARD_SERVICE); 
ClipData clip = ClipData.newPlainText("label", "Text to copy");
clipboard.setPrimaryClip(clip);

For inside the custom (html) webView just add:

let copy = (msg) => {
        let textArea = document.createElement("#"); // name of the element
        textArea.value = msg;
        textArea.style.opacity = "0";
        document.body.appendChild(textArea);
        textArea.focus();
        textArea.select();


        try {
            let successful = document.execCommand('copy');
            let msg = successful ? 'successful' : 'unsuccessful';
            alert(msg);
        } catch (err) {
            alert('invalid', error: ' + err.msg);
            }

            document.body.removeChild(textArea);
        };