Adding fields at the Purchase line item detail windows level

Hi Guys.

I am trying to use extender to add a new window on the Purchase Line item Detail window.  However when creating my window and looking for the form I cannot find the Purchase Line item detail form. all that comes up is:

Purchase order Item and Purchase order itemS

I've tried buy I can't get the extender window to come up on the correct form.

Has any one else tried doing this? can it be done? any help is greatly appreciated.

Thanks,