Force a function to produce a custom HTML code

Try it with get_comment_text() [untested]

<?php
echo '<p id="comment-text">' . get_comment_text() . '</p>';
?>

unless that css id is dependant on the individul comment, you should rather use a css class;

because comment_text() uses a filter for the output:

echo apply_filters( 'comment_text', get_comment_text( $comment_ID ), $comment );

you can try adding a filter function to functions.php, to be able to influence the html paragraph tag;

example to add a css class to the first paragraph tag in the comment text(untested):

add_filter('comment_text','adapt_html_comment_text',30);
function adapt_html_comment_text($text) {
$text = preg_replace( '/\<p/', '<p class="comment-text"', $text, 1 );
return $text;
}

details will depend on what exactly you want to add to the p tag.

Worked! Thanks!