Ok. I found this:
<?php
function sanitize_title_with_urlencode($title) {
$title = urlencode($title);
$title = strtolower($title);
$title = str_replace(‘+’, ‘-‘, $title);
return $title;
}
remove_action(‘sanitize_title’, ‘sanitize_title_with_dashes’);
add_action(‘sanitize_title’, ‘sanitize_title_with_urlencode’);
?>