C#

public partial class Form1 : Form
{
    public Form1(){ InitializeComponent();}

    private Form form2 = new Form();
    private Panel pnl1 = new Panel();
    private Button btn1 = new Button();
    private TabPage tbPg1 = new TabPage();

    private void Form1_Load(object sender, EventArgs e)
    {
        pnl1.BackColor = Color.SlateBlue;
        pnl1.Padding = new Padding(32);

        form2.TopLevel = false;
        form2.FormBorderStyle = FormBorderStyle.FixedSingle;
        form2.ControlBox = false;
        form2.MaximizeBox = false;
        form2.MinimizeBox = false;
        form2.Text = "";
        form2.BackColor = Color.Azure

        form2.Resize += (ob, ea) =>
        {
            btn1.Location = new Point((form2.Width/2) -(btn1.Width/2),
            (form2.Height/2) - (btn1.Height/2));
        };

        form2.Parent = pnl1;
        form2.Dock = DockStyle.Fill;
        form2.Visible = true;

        btn1.AutoSize = true;
        btn1.BackColor = Color.WhiteSmoke;
        btn1.Text = "button on the Form";

        form2.Controls.Add(btn1);
        btn1.Click += (ob, ea) => { MessageBox.Show("Button on Form Clicked"); };

        tbPg1.BackColor = Color.LightSteelBlue;
        tbPg1.Text = "Test Form in Panel in TabPage";
        tbPg1.Padding = new Padding(32);

        tbPg1.Controls.Add(pnl1);
        pnl1.Dock = DockStyle.Fill;

        tabControl1.Controls.Add(tbPg1);
    }
}