Guide to updating the theme for the advanced placement of the app on product pages.
Step 1: Click on Themes in the left-hand navigation and then click on Actions --> Edit code
Step 2: In the theme editor search for product or product-template depending on your theme, below is an example of Shopify's Simple theme product-template.liquid file
Step 3: Insert following code at an appropriate location in product liquid file and save your changes.
Screenshot below shows app widget just above quantity selector in Simple theme