I found a way! :-)
In the file libs/chat.php, function prepare_html_message($text):
Change this line:
$text_w_links = preg_replace('/(http|ftp):\/\/\S*/', '<a href="$0" target="_blank">$0</a>', $escaped_text);
...to:
$text_w_links = preg_replace('/(http|https|ftp):\/\/\S*/', '<a href="$0" target="_blank">$0</a>', $escaped_text);
To hide http/https/ftp:// in the link, change it to this:
$text_w_links = preg_replace('/(http|https|ftp):\/\/(\S*)/', '<a href="$0" target="_blank">$2</a>', $escaped_text);
To allow links with your own text, add the following line after the changed one:
$text_w_links = preg_replace('/(http|https|ftp)link:\/\/(\S*)\*(\S*)/', '<a href="$1://$2" target="_blank">$3</a>', $text_w_links);
To get a link with your own text, add link behind http/https/ftp, add * after the URL and then the text for the link.
E.g: httpslink://yourdomain.com*your_text
...will give the link: <a href=https://yourdomain.com>your_text</a>