How can I make this checking after entering the text in the textbox and pressing the enter key

HI
       in my asp.net C# web page I have a text box and a button. the code in the button click event is
 if (textbox1.Text == "admin")
{
Response.Redirect("AdminHome.aspx"); 
} 
How can I make this checking after entering the text in the textbox and pressing the enter key 
...