How to open forms in xtratabcontrol devexpress

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C#

WinForms

DevExpress

TabControl

Hello
I am trying to open my child forms in tabcontrol that I have put it in main (parent) form insted of for example "center screen" as usual.

here is my code that I have tried :

private void MainForm_Load(object sender, EventArgs e)
{

    AddDocumentManager();
    for (int i = 0; i < 3; i++)
    {
        AddChildForm();
    }
}

void AddDocumentManager()
{
    DocumentManager manager = new DocumentManager();
    manager.MdiParent = this;
    manager.View = new TabbedView();
}
int count;
void AddChildForm()
{
    Form childForm = new Form();
    childForm.Text = "Child Form " + (++count).ToString();

    SimpleButton btn = new SimpleButton();
    btn.Text = "Button " + count.ToString();
    btn.Parent = childForm;

    childForm.MdiParent = this;
    childForm.Show();
}

Thanks in Advance

What I have tried:

I have tried code above and some similar code in Devexpess documents, but could not find the answer