How can I find out the PipelineDeals APP key?

The APP key is a unique application key that will tell Pipeline what integration is making the API request and the API Key is a unique user key that will authenticate the user. 

If you already have an account and are an admin go to the https://app.pipelinecrm.com/admin/modern/api  section of Pipeline to create an APP key and then on the same page, navigate to View API Keys to grab your API key.