A Portal Application in ONAP is a standalone program that can be visually manipulated via the GUI in the Portal. (Note: you can also add a graphical "widget" – in effect a lightweight application) to the Portal UI. To develop such a applications, you will need the development environment of ONAP, such as Java8, along with other tools, depending on the complexity of your application; see Required Tools. Your application will likely need to invoke libraries containing the APIs of the Common Services used with ONAP. Upon completion and testing, you will need to follow the process for submitting an Application to be used in the ONAP environment (see the Certification Group in Mailing Lists).
The following document describes how to create a Portal application in detail:Portal SDK Documentation in referenced documents