[support] How to add "selected" attribute to select list <option> tag

Well, I'm back again, and this time I have a question about attributes for select options in a form. I have a form that has a select element (multi-select) that lists users. The form is for some admin settings for a module. When the form is displayed, I need to be able to highlight specific options in the list based on data returned from a query. I've been looking all day, and haven't been able to figure it out. There is the #attribute key, but it is for the field as whole (at least as far as I can determine), and doesn't work for each individual <option> tag.

If anybody could point me in the right direction, I would appreciate it.

Thanks.

Steve