Requirement - To add a Sales Order via API

My objective is to add an item by inserting "SKU" and not by "Item name" or "Description". 

Note - In the Item Master's UI, I see a field called SKU and I set this to "Item_ID" and I get an error saying invalid product ID. I later queried the "Item_ID" and realised it is not the same as SKU. 

Can someone please guide me as to how to achieve this? Thanking you in advance!

Regards,

Govind