@MohdF Please try the following.

(function runMailScript( /* GlideRecord */ current, /* TemplatePrinter */ template,
    /* Optional EmailOutbound */
    email, /* Optional GlideRecord */ email_action,
    /* Optional GlideRecord */
    event) {

    var assignedTo = current.assigned_to.toString();
	var manageremail = current.assignedTo.manager.email.toString();

    email.addAddress('to', assignedTo);
	email.addAddress('cc', manageremail);

})(current, template, email, email_action, event);