private void bg_loadAccount_DoWork(object sender, DoWorkEventArgs e)
{
myPanel = ...;
myPB = ...;
this.controls.add(myPanel);
this.controls.add(myPB);
myPanel.controls.add(myPB);
}

CheckForIllegalCrossThreadCalls = false; // Разрешить обращаться к контролам из других потоков

CheckForIllegalCrossThreadCalls = false;
Init();

         public MainWindow()
         {
             CheckForIllegalCrossThreadCalls = false;
             Init();
             InitializeComponent();
         }

   
   void WriteAccountInformation(String UserId)
          {
     //куча кода
this.username.Text = FirstName;
          }

          void LoadAccount(string userId)
          {
              CurTarget = userId;

              this.panel_loading.BringToFront();

              this.bg_loadAccount.RunWorkerAsync();
          }

         private void bg_loadAccount_DoWork(object sender, DoWorkEventArgs e)
          {
              Panel loading_panel = new Panel(){Dock = DockStyle.Fill, Location = new Point(0,0), BackColor = Color.FromArgb(255, 51, 62, 75)};
              PictureBox loading_icon = new PictureBox() { Image = Properties.Resources.loading, Location = new Point(0,0), SizeMode = PictureBoxSizeMode.AutoSize };
              loading_panel.Controls.Add(loading_icon);
                
              this.Controls.Add(loading_panel);

              LoadAccountInformation(CurTarget);
          }

          private void bg_loadAccount_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
          {
              WriteAccountInformation(CurTarget);
              this.loading_timer.Enabled = true;
          }