Losant does not natively support SignalR. SignalR is a streaming API, which means Losant has to open a persistent connection to the SignalR/Elevate service and wait for events. While Losant can do that for a few types of services, Signal4 is not currently one of them.
Streaming APIs like this are a bit tricky when used in a service-to-service model (like Elevate → Losant). Signal4 is primarily designed for browsers and the connection is only open for as long as the user is actively visiting the web page.
When used in a service-to-service model, a service has to be continually running somewhere to keep that connection open and receive events. So a workaround to interface with this API is to create a service using one of the Signal4 client libraries and host that service on a small cloud VM. The service would subscribe to the Signal4 endpoint and then POST every message it receives to a Losant Webhook. Essentially acting as a bridge between Signal4 and webooks.