uses
  System.Net.URLClient, System.Net.HttpClient, System.Net.HttpClientComponent;


function PostJson(AUrl : string; APostData : string): string;
var
  APost,
  AResponse : TStringStream;
begin
  APost := TStringStream.Create(APostData, TEncoding.UTF8);
  AResponse := TStringStream.Create('', TEncoding.UTF8);
  try
    with TNetHTTPClient.Create(nil) do
    try
      ContentType := 'application/json';
      Post(AUrl, APost, AResponse);
      Result := AResponse.DataString;
    finally
      Free;
    end;
  finally
    APost.Free;
    AResponse.Free;
  end;
end;

procedure TForm1.Button1Click(Sender: TObject);
var
  AURL,
  APostData : string;
begin
  AUrl := 'https://api.blaulichtsms.net/blaulicht/api/alarm/v1/scenario/config/list';
  APostData := '{"username" : "myUser","password":"mySuperSecretPwd","customerIds" : ["100027"]}';
  ShowMessage(PostJson(AUrl, APostData));
end;