How to enable Save & Add Product button in Opportunity Creation in Salesforce?

Lightning: 
1. Go to Opportunity Settings in Setup.


2. Enable "Prompt users to add products to opportunities". 
 


Classic:
1. Go to Opportunity Settings.


2. Enable "Prompt users to add products to opportunities".


3. When you go to create an Opportunity, you will be able to see "Save & Add Product" button.