could be android:label="@string/app_name" in manifest

HttpStatusCode (the type of response.StatusCode) is an enumeration where the values of the members match the HTTP status codes, e.g.

  public enum HttpStatusCode
  {
     ...
   Moved = 301,
   OK = 200,
   Redirect = 302,
    // Summary:
    //     Equivalent to HTTP status 202. System.Net.HttpStatusCode.Accepted indicates that
    //     the request has been accepted for further processing.
    Accepted = 202,
    //
    // Summary:
    //     Equivalent to HTTP status 400. System.Net.HttpStatusCode.BadRequest indicates
    //     that the request could not be understood by the server. System.Net.HttpStatusCode.BadRequest
    //     is sent when no other error is applicable, or if the exact error is unknown or
    //     does not have its own error code.
    BadRequest = 400,
    ...
 }

The steps to find the question is as follows:
1. When we debug the request, we will find the following response :

{ StatusCode: 400, ReasonPhrase: 'BadRequest', ....... }

Since the StatusCode: 400, so the request of you code could not be understood by the server.
When we test the api by using Postman, we can see the message as following image :

From the capture image, we will find the Message:

{"Message":"Non-static method requires a target."}

"Non static method requires a target" means that some object inside the Json is null.
2. When I add the requested field as follows according the api you provided, the error disappeared.

    public class SenderStatus
{
    public string Name { get; set; }
    public int ID { get; set; }   // new field
}

public class RegisterSender
{
    public string Name { get; set; }
    public string SocialID { get; set; }
    public string Phone { get; set; }
    public double RatingAvg { get; set; }
    public int SenderID { get; set; }   // new field 
    public SenderStatus senderStatus { get; set; }
}

and use the following code to test the api

        RegisterSender registerSender = new RegisterSender();
       registerSender.Name = nametext.Text;
       registerSender.Phone = phonetext.Text;
       registerSender.SocialID = idText.Text;
       registerSender.RatingAvg = Convert.ToDouble(ratingText.Text);
        registerSender.SenderID = 88;   //  replace the value yourself 

        SenderStatus statusModel = new SenderStatus();
        statusModel.Name = "Active";
        statusModel.ID = 78;      // replace the value yourself 
        registerSender.senderStatus = statusModel;

when we debug the code, the response is as follows

 {
StatusCode: 200,
ReasonPhrase: 'OK',
Version: 1.1,
Content: System.Net.Http.StreamContent,
Headers: {
   //.........
 }
}

The capture is

Since StatusCode :200 , so the request is successful.
3. Besides, the code you use to verify the request result is not correct.
You can use the following code.

        bool success = response.IsSuccessStatusCode;
        if (success)
        {
            Toast.MakeText(this, "Sender created succesfuly", ToastLength.Short).Show();
        }
        else
        {
            Toast.MakeText(this, "Something went wrong", ToastLength.Short).Show();
        }

For more details, you can check:https://stackoverflow.com/questions/1330856/getting-http-status-code-number-200-301-404-etc-from-httpwebrequest-and-ht

Note:
I strongly suggest you to check the request api with the Background server by using the Postman.
For more details: https://www.c-sharpcorner.com/article/how-to-use-postman-with-asp-net-core-web-api-testing/

You can try the following code:

        Toast toast = Toast.MakeText(this, "test", ToastLength.Long);
        toast.SetGravity(GravityFlags.Center, 0, 0);
        toast.Show();