This paper presents the definition and a proof-of-concept implementation of an intent-based northbound interface (NBI) used to orchestrate dynamic service chaining of Virtualized Network Functions (VNFs) by actively controlling the underlying network infrastructure through vendor-independent, technology-agnostic policies. The proof of concept considers a general scenario where VNFs are hosted in different SDN domains, possibly interconnected by non-SDN domains. Being implemented as part of a Virtualized Infrastructure Manager, the proposed NBI is compliant with the ETSI NFV management and orchestration specifications, as well as with the recent ONF definition of intent-based NBI. The case of a network operator that provides adaptive quality of service enforcement in a multi-tenant scenario is considered. Responsiveness of the intent-based NBI is experimentally evaluated under increasing load, proving the correct functionality and the scalability potentials of the proposed approach.