My question is where to copy the short code you provided??? This is in reference to the two posts as follows

From 25035 above
Is there a way to make this so when a user replies to a group message, can choose to reply to whole group or reply to person who sent group message only?

and from 25050 above

You can use shortcode https://www.shamimsplugins.com/docs/front-end-pm/shortcode/fep_shortcode_message_to/ to show a button above reply form. When someone will click that button will redirect to message page with “To” field pre-populated with sender name.

My users are very unhappy with so many replies to a group message. Where do I copy this code to provide a button above the reply form?? Thank you