Using Internet Explorer you cannot add the smart tags to the email by clicking on the boxes below the body of the message. There is no scheduled update for Internet Explorer since IE has been discontinued since August 2021.
However, you can copy-paste the wording of the smart tags for the specific event from another browser or type them yourself in the text area using brackets, e.g., {related_user_email}, {related_user_name}, {site_url}, etc.
Please note that smart tags are also functional using Microsoft Edge, the updated version of Internet Explorer, in case you are interested in replacing it but are not fans of Mozilla Firefox or Google Chrome.