Hello,
I think I found the problem: there are 2 ways to insert a dropdown, and they don't have the same properties.

Here are the 2 properties windows:
http://static.ddmcdn.com/gif/dropdown-form-field-2.jpg
http://www.79xperts.com/blog/wp-cont...creenshot4.jpg

So now I can set the bookmark in the dropdown properties.
As soon as I have time to re-try the code of @macropod, I'll give news.

Thank you again!