Just tested with WordPress 4.9.4 and it is working!
Copy paste my contact form which has been tested:
[hidden ts_sendy_hidden “true”]
[tssendy listID “7914”]
[text* ts_Name placeholder “name”]
[email* ts_Email placeholder “email”]
[submit “go”]
Make sure you change 7914 to your list ID. Also, double check the Sendy Installation URL in plugin settings. I forgot to add https:// and that was the reason it was not working in the beginning.