It depends of what you are trying to do.

Let’s say that, in App B, your user will send a “product” to your App A.
And in App A, you will pick this product and save it in your database. Very simple example.

So, in App A you will need to create an API Workflow to receive and save the incoming product in your database, correct? Go to your backend workflows and create something like this:

You can see that this API workflow needs a variable. A product. Text type. Now, when you receive this product, just create a workflow to save it in your database.

When you open you App Connector plugin in App B, you should be able to see the above workflow (product_from_appb) in a list of workflows. Now, you can send a product thru this workflow from App B to App A. And as we already saw, when this workflow receives a product, it will save it in its database.