Creating the form element. Populate a list of lists with values for the option list.

set foo_options [db_list_of_lists foo_option_list "
    select foo,
           foo_id
      from foos
"]

The variable foo_options should resemble {{first foo} 1234} {{second foo} 1235}

Within ad_form, set up the element to use this list:

{foo:text(select)
        {label "Which Foo"}
        {options $foo_options}
    }

This will result in a single name/value pair coming back in the submitted form. Handle this within the same ad_form structure, in the -new_data and -edit_data. In the example, it is available as $foo