Thank you very much. 
It works exactly what I am looking for. 
Just out of curiosity, 
Do you think I can add
{{#if this.user.can_send_private_message_to_user}}
to “pm-topic-author.hbs” , so that the button won’t show up if the user is the same as the OP ? 
Similar to Discourse User card “user-card-contents.hbs”
