You can use one of the following hooks:

• comment_main_form_after_head
• comment_form_top
• wpdiscuz_comment_form_top
• comment_form_before
• wpdiscuz_comment_form_before

The example is provided below:

add_action("comment_main_form_after_head", function () { 
    echo "Custom Message"; 
});

The code should be added in your active theme functions.php file.