public static string FormatOrderCode(int orderId, int regionCode)
{
    // Allocates exactly 10 characters with 0 intermediate StringBuilder or string allocations!
    return string.Create(10, (orderId, regionCode), static (span, state) =>
    {
        span[0] = 'O';
        span[1] = 'R';
        span[2] = 'D';
        span[3] = '-';
        state.regionCode.TryFormat(span.Slice(4, 2), out _, "D2");
        span[6] = '-';
        state.orderId.TryFormat(span.Slice(7, 3), out _, "D3");
    });
}