Another possibility apart from App Studio mentioned by @Gurpreet. is for the 3rd party to have a Windows application that could use Eikon Data APIs if it needs to retrieve data from Eikon and Side by Side Integration API if it needs to interact with Eikon application.
To join App Studio program a partner needs to find a business sponsor within Refinitiv (a head of a business vertical willing to sponsor partner's participation in App Studio program). No sponsorship is required to develop a Windows application using Eikon Data APIs and/or Side by Side Integration API. However the 3rd party will need an Eikon account to use any Eikon APIs. We may offer Eikon to a 3rd party strictly for software development purposes at 25% of the list price.

@radhika.deshpande A developer needs to have a working Eikon installation for app development. There is a quick start on the developers portal which introduces the App Studio. All the tutorials and samples are available within Eikon itself. The first step to get going is to get a working Eikon terminal.