TabPage tp1 = new TabPage();
tabControl1.Controls.Add(tp1);
tp1.Text = "New tab";
WebBrowser wb1 = new WebBrowser();
tp1.Controls.Add(wb1);
wb1.Parent = tp1;
wb1.Dock = DockStyle.Fill;
wb1.Navigate(defaultUrl);
wb1.ScriptErrorsSuppressed = true;
wb1.DocumentTitleChanged += Wb1_DocumentTitleChanged;
wb1.DocumentCompleted += Wb1_DocumentCompleted;

private void textBox_KeyDown(object sender, KeyEventArgs e)
{
    if (e.KeyCode == Keys.ENTER)
    {
        WebBrowser wb1 = sender as WebBrowser;
        wb1.Navigate(textBox.Text);
    }
}

var browser = (WebBrowser)new TabControl().SelectedTab.Controls[0];
browser.Navigate(textBox.Text);