SendMessageController.cs

using System;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Mvc;
using NServiceBus;
using NServiceBus.TransactionalSession;

[ApiController]
[Route("")]
public class SendMessageController : Controller
{
    private readonly MyDataContext _dataContext;

    public SendMessageController(MyDataContext dataContext)
    {
        _dataContext = dataContext;
    }
    
    [HttpGet("/")]
    public async Task<string> Get([FromServices] ITransactionalSession session)
    {
        var id = Guid.NewGuid().ToString();
        
        await _dataContext.MyEntities.AddAsync(new MyEntity { Id = id, Processed = false });
        
        var message = new MyMessage { EntityId = id };
        await session.SendLocal(message);
        
        return $"Message with entity ID '{id}' sent to endpoint";
    }
}