How to use the yagmail.headers.add_subject function in yagmail

contents = attachments if contents is None else contents + attachments

    has_included_images, content_objects = prepare_contents(contents, encoding)
    msg = MIMEMultipart()
    if headers is not None:
        # Strangely, msg does not have an update method, so then manually.
        for k, v in headers.items():
            msg[k] = v
    if headers is None or "Date" not in headers:
        msg["Date"] = formatdate()

    msg_alternative = MIMEMultipart("alternative")
    msg_related = MIMEMultipart("related")
    msg_related.attach("-- HTML goes here --")
    msg.attach(msg_alternative)
    add_subject(msg, subject)
    add_recipients_headers(user, useralias, msg, addresses)
    htmlstr = ""
    altstr = []
    if has_included_images:
        msg.preamble = "This message is best displayed using a MIME capable email reader."

    if contents is not None:
        for content_object, content_string in zip(content_objects, contents):
            if content_object["main_type"] == "image":
                # all image objects need base64 encoding, so do it now
                email.encoders.encode_base64(content_object["mime_object"])
                # aliased image {'path' : 'alias'}
                if isinstance(content_string, dict) and len(content_string) == 1:
                    for key in content_string:
                        hashed_ref = str(abs(hash(key)))
                        alias = content_string[key]