BS: Create the dropdown as copy from another Data window
dw_1.item.myfield.dropdown.fromdw=dw_2,fieldId,fieldDisplay
In dw_2 the follow fields must exists: fieldid and fielddisplay
Example:
in dw_2 you have itemcode and itemname and want to display as dropdown the itemname as dropdown in dw_1.myitems
dw_1.item.myitems.dropdown.fromdw=dw_2,itemcode,itemname