I found a response following this thread:
https://openacs.org/forums/message-view?message_id=120754In details here:
http://cvs.openacs.org/cvs/openacs-4/contrib/packages/project-manager/
In task-add-edit.tcl .
So I changed my code:
ad_form -name filtri_bk -form {\
{bk_fatture_f:text(select) \
{label bk_fatture_f} \
{options {{10 10} {20 20}}} \
{value t }}}
And finally the result was:
<select name="bk_fatture_f" >
<option value="10" selected="selected">10</option>
<option value="20">20</option>
</select>
Wow!