You can use https://www.shamimsplugins.com/docs/front-end-pm/shortcode/fep_shortcode_message_to/ . It will show button, clicking that button will redirect you to message page with “To” auto filled.
OR
https://www.shamimsplugins.com/docs/front-end-pm/shortcode/fep_shortcode_new_message_form/ . It will show a form and from product page users can send message to product seller.
Or if you use any code, you can pass fep_to=user_nicename to auto fill “To”