Use the Context object in the hub to get the ASP.Net Core Identity UserId.
var userId = Context.UserIdentifier;
https://learn.microsoft.com/en-us/aspnet/core/signalr/hubs?view=aspnetcore-9.0#the-context-object:~:text=before%20SendAsync%20finishes.-,The%20Context%20object,-The%20Hub%20class