Hi,
I tend to use a macro for this, something along the lines of:
Define Fill_Po. ls_po_Header-&1 = &2. ls_po_Headerx-&1 = Abap_True. End-Of-Definition
And then when you want to populate your structure:
Fill_Po: Po_Number '9000000020', Comp_Code '1000', Doc_Type 'ZFIT'.
Regards,
Rich