Method overloading C#

I'm new to C# and trying to figure out how i can easily implement the following:

I have a subscriber:

public interface ISubscriber
{
    void HandleMessage(IMessage message);
}

class TriggerSubscriber : ISubscriber
{
    public void HandleMessage(IMessage message)
    {
         //This will be called by default   
    }
    public void HandleMessage(TriggerMessage message)
    {
         //This won't work, why?    
    }
}

Trying to handle the following message:

public class TriggerMessage : IMessage
{
    string identifier { get; set; }

    public TriggerMessage(string triggerIdentifier)
    {
        identifier = triggerIdentifier;
    }
}

public interface IMessage
{

}

I expected that the following would have been called:

public void HandleMessage(TriggerMessage message)

instead of

public void HandleMessage(IMessage message)

with

ISubscriber trigger = new TriggerSUbscriber();
trigger.HandleMessage(
    new TriggerMessage()
);

Am i missing something here?