Insert PO in 541 to reservation in MB1B

Hi Gurus,

I have a problem when I make a good movement with 541 O by MB1B.

I posted a reservation by MB21 with good movement 541, then I needed to post a 541 O movement by MB1B to reservation with indication of a PO.

I have in customizing set in OMBW the field EBELN as optional entry, but when I tried to post the document by MB1B to reservation, the system not displays this field.

How can I do?

Thanks

Nik