Create a rule in the inv workflow spreadsheet that contains the field PO nbr showing in the inv line

I want to create a new rule in the invoice approval workflow spreadsheet that contains the PO number showing in the invoice line and not header.

My conditions:

-First: if the PO number showing in the invoice line is not null --> auto approve

-Second: if the PO number showing in the invoice line is null --> invoice header requester

I cannot find the field of PO number (invoice line) in this spreadsheet.