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.

<div id="inventory-info-app" class="iia-container"></div>

Screenshot below shows app widget just above quantity selector in Simple theme