Do you want a member to be notified that you made a reply to them? “Mention” them in the reply! Use the “@“ symbol and their USERNAME (this may not be their screenname…you may need to click on their profile and look on their cover photo and then click back). Type @username and your reply and it will show up in their notifications and they will know you mentioned them.
‘It is unnecessary to type @username into a reply if you want the AUTHOR of the post to know you replied. All authors of posts get notifications of all replies.