It's in the Answer File on the Network Identification page (Step 13 in the below link).
https://support.smartdeploy.com/support/solutions/articles/48000960920-create-an-answer-file
It uses the Distinguished Name found on the Attribute Editor tab in you AD listing. Open it up to the editor, copy the entire line (without the computer name) and paste into the OU field.
Hope this helps!