You can do this by customizing the template that is used to send the WordPress content to Discourse. There is some information about how to do that here: WP Discourse template customization
Adding something like this to your functions.php
file should work. Insert whatever html you want to use between the closing and opening php tags.
function your_namespace_publish_format_html() {
ob_start();
?>
<p>Generic content to be published to Discourse with every WordPress post...</p>
<?php
$output = ob_get_clean();
return $output;
}
add_filter( 'discourse_publish_format_html', 'your_namespace_publish_format_html' );