"Avdelning" section in Voucher Lines

Hi, so i wanted to add the information something like the department or section ("project" or cost place") on every voucher entry. How do i do that? Currently the voucher lines only accept account, amount, text currency. i enabled the avdelning on every voucher line entry but i have no idea how to access that field from the REST API. is there any way i can attach that information (project or costplace) on every voucher line? here is the screenshot of the voucher http://screencast.com/t/kwBkM6Cb Thank you for your help.