Textbox value in other textbox by clicking button not correct

Textbox value in other textbox by clicking button not correct

This program have a link which is fixed and never change. And it contains
5 textboxes. The fixed link is:
<seite>utm_source=<website>_<page>_de&utm_medium=banner&utm_campaign=<kampagne>&utm_content=<format>
Every value in <> should be changed by textbox value. Here you got an
image of my little program:

Now my problem is: the first value is correct, but the other values
aren't. So for example, if i type in second texbox: "website" it does not
only replace <website> with "website". it replaced <website> with
System.Windows.Forms.TextBox, Text: website.
My Code I tried:
private void btn_SendAll_Click(object sender, EventArgs e)
{
txt_FinishLink.Text = txt_Site.Text + "utm_source=" +
txt_Website + "_" + txt_Page +
"_de&utm_medium=banner&utm_campaign=" + txt_Campaign +
"&utm_content=" + txt_Format;
}