Populate Select List from external tables 

In order to populate a selectbox field with values taken from an external table, you'll have to paste this script in the Scripts called on form Display, and change col1, col2 and my_external_table to match your needs.

foreach($fields as $i=>$field){
    if($field->name=='select1'){
        $database->setQuery("SELECT col1 as val, col 2 as txt FROM my_external_table");
        $values = $database->loadObjectList();
        $default_value = array();
        foreach($values as $row){
            $default_value[]= $row->val.'|'.$row->txt;
        }
        $default_value = implode(',',$default_value);
        $fields[$i]->default_value = $default_value;
    }
}