Firebase Cloud Messaging content is not sending

I'm trying to send a notification via an back-end server that I made and I think my content that I send with the call is incorrect.

var client = new HttpClient();
var request = new HttpRequestMessage(HttpMethod.Post, "https://fcm.googleapis.com/fcm/send?project_id=bespaargroen-370414");
request.Headers.Add("Authorization", "Bearer AAAA4u_fn_I:MyKey");
var content = JsonConvert.SerializeObject(not, Formatting.Indented);
request.Content = new StringContent(content);

var response = client.Send(request);
response.EnsureSuccessStatusCode();
var repsononej = response.Content.ReadAsStringAsync();

return new BoolResponse()
{
    Success = true,
    Message = "Bericht verzenden gelukt"
};

the content variable leads to a model i made:

public class NotificationData
{
    [JsonPropertyName("title")]
    public string Title { get; set; }
    [JsonPropertyName("body")]

    public string Body { get; set; }

    public string from { get; set; }
    public bool read { get; set; }
    public string to { get; set; }
    public bool mutable_content { get; set; }
    public string sound { get; set; }
}

now the question is: how should i do it properly. because I want to connect an easy UI to this so i can send messages when i want it and how i want it.

I do not get any errors back. the message i send just goes to the void.

It is probable that there is a problem with the device token that you are using.
It could be no longer valid, or incorrect.

NOTE: You might find that the phone app must be closed for it to receive the notification.
Also, you must ensure that the app on the phone can actually receive notifications. When I install my test apps locally (not via Google play), then by default notifications are blocked.

But I would suggest to use the FirebaseAdmin nuget API to simplify you interaction with the API.
Then you can do something like this.

var instance = FirebaseApp.Create(new AppOptions
    {
         //or however you want to load it
        Credential = GoogleCredential.FromFile("firebase_key.json")
    });

//If you use MulticastMessage() then you can send a list of tokens
var message = new Message()
{
    Data = new Dictionary<string, string>()
    {
        {"myData", "detail sent in message" },
    },
    Token = "token id for the specific device",
    //Topic = "my topic", // only if not targeting a specific device(s)
    Notification = new Notification()
    {
        Title = "Test from code",
        Body = "Body of message is here",
    },            
};

string response = FirebaseMessaging.DefaultInstance.SendAsync(message).Result;