The above image shows the page where an Admin creates a new product.
When the user clicks the Add file button (1) and the image gets uploaded. It should trigger a function in my app and pass the uploaded picture to it. My app will thereafter make a simple API call and return a simple string. This string should to autofilled in the product title (2).
Can anybody guide me on how to accomplish this task?
I don't think that would be possible using the Shopify API. You could get a webhook event for when a product is updated, but the merchant would need to 'save' the product, not just upload an image.