Auto populate a Form

add_filter( 'comment_form_defaults', 'bootstrap3_comment_form' );

function bootstrap3_comment_form( $args ) {
    $args['comment_field'] = '<div class="form-group comment-form-comment">
    <textarea class="form-control" id="comment" name="comment" cols="45" rows="8" aria-required="true" placeholder="' . __( 'Comment', "beautiful" ) . '"></textarea>
</div>';
return $args;
}

fetch your content and put it inside textarea tag.

you should add this function to your functions.php it’s better to change name of the function and language text domain.

this function override WordPress default textarea of comment section. it’s also with bootstrap 3