You are onboarding an account that is not supported out of the box.
What should you do first to obtain a platform to import?
Create a service ticket in the customer portal explaining the requirements of the custom platform.
Search common community portals like stackoverflow, reddit, github for an existing platform.
From the platforms page, uncheck the “Hide non-supported platforms” checkbox and see if a platform meeting your needs appears.
Visit the CyberArk marketplace and search for a platform that meets your needs.
Submit