There is currently a wrapper REST API available for RELEVANCE search, that the platform itself uses. It appears to be unsupported for external use.
It would be great if a Relevance Search API was made available outside the platform, so it can be used from an external custom Web Site / Portal.
Currently Relevance Search is configurable for an entity and most text fields within Dynamics 365 platform via Quick Find for the Entity. But it would also be great to make it configurable by location too, such as data which may need to be searched upon external to the platform. For example this would prove very beneficial for knowledge base search, where Knowledge Articles which have been marked as available on a Portal can be searched by customers.