Why am I not able to go to the implementation of a method in an external assembly instead of just the interface?

I have this line of code that uses Mediatr:
        var vm = await Mediator.Send(new GetCustomerDetailQuery { Id = id });

Mediatr is an external package. I  am trying to go to the implementation of the Send method which is in Mediatr. I tried ctrl-click, ctr-alt -click. Keyboard short cuts. It goes to the IMediator interface.
It also shows two interfaces and if I select either, II get implementations of Send not found.

The implementation should be in the Mediatr assembly. Why do R# and Rider not go to the actual implementation and show implementstion not found?