I hope you have added the field in Item Journal Line table and Item Reclass. Journal page.
Posting happens in the codeunit Item Jnl.-Post Line" so use the below event
OnBeforeInsertItemLedgEntry
[EventSubscriber(ObjectType::Codeunit, Codeunit::"Item Jnl.-Post Line", 'OnBeforeInsertItemLedgEntry', '', false, false)]
local procedure OnBeforeInsertItemLedgEntry(var ItemLedgerEntry: Record "Item Ledger Entry"; ItemJournalLine: Record "Item Journal Line"; TransferItem: Boolean; OldItemLedgEntry: Record "Item Ledger Entry"; ItemJournalLineOrigin: Record "Item Journal Line");
begin
ItemLedgerEntry.SelectOption := ItemJournalLine.SelectOption;
end;
