Mock a Func and Return values for Unit Test C#

public class SqlUserProvider : SqlProvider<User>, IUserProvider
{            
  public User FindUserName(string userName, Client client)
  {
    return User
  }   
}    

public interface IUserProvider:ISecurityProvider<User>
{
   User FindUserName(string userName, Client client);      
}

[HttpGet]
public HttpResponseMessage Login()
{
   User user = nul
   user = _userProviderProxy().FindByUserName(username, client);
   user.IsAuthenticated = success;           
   return response;
}

How to create Mock for _userProviderProxy().FindByUserName(username, client),

I try to create Mock for IUserProvider, but return is not working, I want return a user after mock.

I try to Mock like this

_proxy.Setup(x => x.FindByUserName(It.IsAny<string>(), It.IsAny<Client>()))
    .Returns(new User {Id=1})

but it is not working