No need for the field. Just add the Products::ProductName field to your Order Items portal row or layout. When you enter or select a value in kf_productID, the product name will automatically appear in the Products::ProductName field.
Note: with this setup, if you edit a Product Name in Products, the ProductName field for that Product will automatically update to match in all existing orders. If you don't want this update to occur (Some businesses prefer not to see past orders update like this), you'd define a text field in OrderItems and use a Looked Up value field option to copy the ProductName from Products into this field.
Thank you Phil,
I used the first option method of just adding the field to the portal.
So this is what I did:
In Order Layout -> Created Order Items Portal -> added Products::ProductName field in portal.
The product name is definitely showing up. It is all working now.