Event on new sales line

Hey Dynamics users,

i'm working on a customization, that needs to be implemented using events. to keep it short, i have a list of items that has a relation to one item. For example, itemA has a relation to ItemB etc.  on a sales order, when a choose ItemA, i also want Item be to appear below it. I'm am a litte bit in doubt on which event i need to subscribe. OnAfterInsert or OnAfterValidate.  I hope someone will guide me to a solution. Thank you.