Inserting item in order via SQL

I need to insert an item in new orders according to certain conditions via SQL (it's a Christmas gift to all orders from 15th November on).
I'd like to use a trìgger in Order table for insert, but the problem is that I don't know what to insert into OrderItemGuid field in OrderItem table.
Can anyone help me please?
Thanks a lot

Hi. You can use NEWID() T-SQL function to generate the ID.

Great, thank you very much!