Hi @SebastianGerdes,
Forge does not yet have the capability for an app to expose its own REST API. We are tracking suggestions and features requests in the FRGE project if you would like to create one.
In the meantime, you could investigate using a web trigger module to expose a URL that can be invoked for your Forge app (giving you the ability to mimic an API for your app). Keep in mind, though, that web triggers are not authenticated and may not be suitable if you need to verify the identity of the client.
Alternatively, we have release alpha support for adding Forge and Connect functionality within a single app (see our announcement). You might be able to use this approach to migrate some of your functionality to Forge while still relying on the Connect parts of your app to expose the REST endpoint.
Hope this helps! Happy to chat further about either of these options if you would like more detail.
Regards,
Joe