I need to track purchases in GA4. I am using the Google Analytics Measurement Protocol.

I have a listener that is a web server written in .Net Core. Everytime a payment happens in my platform, the payment provider call my listener where I register the payment. Contextually, I need to track the payment on GA4.

Here the code I am using:

public async Task TrackPaymentAsync(string measurementId, string apiSecret, PaymentData paymentData)
{
    string url = $"https://www.google-analytics.com/mp/collect?measurement_id=${measurementId}&api_secret=${apiSecret}";

    string requestData = $@"
        {{
        ""client_id"": ""{paymentData.GAClientId}"",
        ""user_id"": ""{paymentData.UserId}"",
        ""events"": [{{
            ""name"": ""purchase"",
            ""params"": {{
            ""currency"": ""EUR"",
            ""value"": ""{paymentData.Amount}"",
            ""transaction_id"": ""{paymentData.TransactionId}"",
            ""debug_mode"": true
            }}
        }}],
        ""user_data"": {{
            ""amount"": ""{paymentData.Amount}"",
            ""fee"": ""{paymentData.Fee}"",
            ""item_name"": ""{paymentData.ItemName}"",
            ""payment_method"": ""{paymentData.PaymentMethod}""
        }}
        }}
    ";

    using (HttpClient client = new HttpClient())
    {
        var content = new StringContent(requestData, Encoding.ASCII, "application/json");
        HttpResponseMessage response = await client.PostAsync(url, content);

        response.EnsureSuccessStatusCode();

        //string responseBody = await response.Content.ReadAsStringAsync();
    }
}

The payload is a mix of the examples in Send user-provided data and Verify implementation .
I get a HTTP 204 but nothing happen on GA4.

I have some doubts:

1. First of all, I got the measurement id from GA4 -> My prop -> Admin -> Data Streams . The doubt I have is the stream url… it is different from the listener url. The listener is “outside” my domain. I also would like to test the listener from localhost. Is it possible?
   
2. The guide says: You must use tagging (gTag, Tag Manager, or Google Analytics for Firebase) to use this protocol.. I am not using any of them. Is it strictly required use one of them?

Can anyone help me please?

Thank you