Response status code does not indicate success: 400 (Bad Request)

Hi! Getting error 400. My code:
 

public async Task TrackEventAsync(string userId, string eventType, object eventProperties)
{
    var eventData = new
    {
        api_key = _apiKey,
        event_type = eventType,
        user_id = userId,
        event_properties = eventProperties
    };

    var json = JsonSerializer.Serialize(eventData);
    var content = new StringContent(json, Encoding.UTF8, "application/json");

    try
    {
        var response = await _httpClient.PostAsync("https://api.amplitude.com/2/httpapi", content);
        response.EnsureSuccessStatusCode();
        _logger.LogInformation("Event tracked successfully: {EventType}", eventType);
    }
    catch (HttpRequestException ex)
    {
        _logger.LogError(ex, "Error sending event to Amplitude: {EventType}", eventType);
    }
}

Any ideas how to figure out the issue?