Are you searching for the endpoint GetCustomFields?
You might wonder why there isn't an endpoint called GetCustomFields present in the API. The reason behind this is that Custom Fields is part of the response when getting a entity through the API. If you choose to include it.
The nature of a Custom Field is such that it´s not aware of what type the parent is. But the parent is aware of its Custom Fields. Thus it dont make much sence fetching a Custom Field without knowing its parent.