Prat,
Follow the steps below to add new field.
1. Go to Purchase Order WC
2. Click on create new purchase order
3. Click on Adapt on the top right corner of your screen-->enter adaptation mode-->yes to the pop up and ok. Click on adapt and edit screen.
4. Highlight General information under Sections and click on extension field-->add--> Type in service in Field label, Tooltip and technical name and save. Leave the rest empty.
5. Tick the displayed box service under fields and publish. If you want the service to be displayed in reports go to further usage--> add the service to the data sources and reports tabs.
6. Publish and overwrite any personalization settings for that screen that user might have saved.
7.Go to adapt and leave adaption mode.
Hope this will help you.
Regards,
Emeka