Before TableB::ItemnumberID gets a value, it is not linked to any record in TableA and the value of TableA::Itemnumber is not accessible from the context of that new record. Use a variable to carry the value to the new record and link it correctly.
Set Variable [$Item ; value: TableA::ItemNumber ]
Go to Layout ["TableB" (TableB)]
Set Field [TableB::ItemNumber ; $Item ]
That worked great. Thanks