how to make SalespersonID equal Current user ID by default when SalespersonID is null

Put this is a Graph Extension of SOOrderEntry:

protected virtual void _(Events.FieldDefaulting<SOLine, SOLine.salesPersonID> e, PXFieldDefaulting b)
{
    SOLine row = e.Row;
    if (row is null) return;

    // Execute regular defaulting logic first
    b?.Invoke(e.Cache, e.Args);

    // If the SalesPersonID is still null, default to the current user's SalesPersonID
    if (row.SalesPersonID == null)
    {
        SalesPerson currentSalesPerson = SelectFrom<SalesPerson>.
            InnerJoin<EPEmployee>.
                On<EPEmployee.FK.SalesPerson>.
            InnerJoin<Users>.
                On<EPEmployee.FK.User>.
            Where<Users.pKID.IsEqual<AccessInfo.userID.FromCurrent>>.View.Select(e.Cache.Graph);

        e.NewValue = currentSalesPerson?.SalesPersonID;
        e.Cancel = true;
    }
}

PX.Objects.SO.SOOrderEntry