Welcome @Francesco_de_Minici1,
Check out this Forum Post here: Looking up values in custom contact field with REST V2 API
The XML-RPC API has the flexibility to do what you want, but we do not know when they plan to sunset it. Keap need to be do a lot more work with the REST API, including being able to query Custom Fields. Otherwise there is going to be gaps with the REST API that will make it difficult to transition across in the future.
The REST V2 API is still being developed, but there is no documentation for the Tasks yet.
https://developer.keap.com/docs/restv2/#tag/Task
@Tom_Scott can give a better response to this.