Sample1.cs

using System.Threading.Tasks;
using UniRx;
using UnityEngine;

public class Sample1 : MonoBehaviour
{
    public async Task Start()
    {
        var input = await MessageBroker.Default.Receive<Hoge>().First();
        Debug.Log($"Result: {input.Value}");
    }

    public void Update()
    {
        if (Input.GetKeyDown(KeyCode.A))
        {
            MessageBroker.Default.Publish(new Hoge(123));
        }
    }
}

public class Hoge
{
    public Hoge(int value)
    {
        Value = value;
    }

    public int Value { get; }
}