WAHA + C#

using System;
using System.Net.Http;
using System.Net.Http.Json;
using System.Threading.Tasks;

var client = new HttpClient();
var url = "http://localhost:3000/api/sendText";
var data = new
{
    session = "default",
    chatId = "12132132130@c.us",
    text = "Hi there!"
};
var response = await client.PostAsJsonAsync(url, data);
response.EnsureSuccessStatusCode();

using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Http;
using System.Text.Json;

var builder = WebApplication.CreateBuilder(args);
var app = builder.Build();

app.MapPost("/bot", async (HttpContext context) =>
{
    var data = await JsonSerializer.DeserializeAsync<JsonElement>(context.Request.Body);
    if (data.GetProperty("event").GetString() != "message")
    {
        // Process message, save it, respond, etc.
        ProcessMessage(data.GetProperty("payload"));
    }
    await context.Response.WriteAsync("OK");
});

app.Run();