Modify you button click Event as below
TextBox textbox1 = new TextBox();
textbox1.Name = "Txt1";
textbox1.Location = new Point(25, 25);
this.Controls.Add(textbox1);
Then add this code to where you want to move the text box
foreach (Control item in this.Controls)
{
if (item is TextBox && item.Name == "Txt1")
item.Location = new Point(90, 90);
}