You can find material on this link, when you create the field to populate it with the sql statement choose the connection you made and then write the sql on the box below.
The final xml will be something like>
<MyDropdown type="dropdown" required="0" readonly="0" savelabel="0"
mode="edit" options="Array" sqlconnection="dbarray">
SELECT CONTACT_ID, CONTACT_NAME FROM CONTACTS
<en>Select Contact</en>
</MyDropdown>
Regards.