The dutch part of the forum is not here yet, so I have my answern in English:
You want to make a link with a tooltip. Here is an example of a link for the sitemenu.ini:
linkx =
www.photo.gallery (this is the buton text)
linkxrollover = tooltip (function)
linkxrolloverdata = Click here to visit <b>www.photo.gallery</b> (text in tooltip)
linkxclick = _blank (open new window, when clicking)
linkxclickdata =
https://www.photo.gallery (url)
set linkxclick to false (linkxclick = false) if you don't want to go to another page/site.