Assuming:
1. you have a TabControl named 'tabControl1 on a WinForm
2. 'tabControl1 has two TabPages that you've named tabXML and tabText
3. 'tabXML has a TextBox inside it (multiline set to 'true) named 'tbXmL
4. 'tabText has a TextBox inside it (multiline set to 'true) named 'tbText
5. you have two Buttons, 'loadXML, and 'loadText connected to these Click EventHandlers:
private void btnLoadXML_Click(object sender, EventArgs e)
{
tabControl1.SelectedTab = tabXML;
tbXML.Focus();
}
private void btnLoadText_Click(object sender, EventArgs e)
{
tabControl1.SelectedTab = tabText;
tbText.Focus();
}