@Ram_Gopal_Rao AFAIK, Same functionality was developed for email alert
You can fork and add this feature for standard reply so it will be helpful for all.
If you want quick fix, you can addd your custom standard reply by overriding existing script as follows.
frappe.views.CommunicationComposer = frappe.views.CommunicationComposer.extend({
make_fun: function(){
var me = this;
var content_field = me.dialog.fields_dict.content;
var subject_field = me.dialog.fields_dict.subject;
var rec = me.dialog.fields_dict.recipients.get_value();
recipients_field.set_input(rec +"Custom@gmail.com")
},
make: function(){
this._super();
this.make_fun();
}
});`
Regards,
Sambhaji Kolate,