I decided to write this article because I always encounter this kind of problem in the ASPNET Forum frequently. So here's a simple (one way) solution on how to invoke the Button Click event when pressing the ENTER key in the TextBox control.

ASPX Mark Up and JavaScript function

<head>
    <title>Untitled Page</title>
</style>
<script type="text/javascript" language="javascript">

 function controlEnter (obj, event)
 {      
     var keyCode = event.keyCode ? event.keyCode : event.which ? event.which : event.charCode;      
     if (keyCode == 13)
     {                  
        document.getElementById(obj).click();
                    return false;      
     }      
     else  {
                   return true; 
     } 
 }
</script>
</head>
<body>
    <form id="form1" runat="server">

        <asp:TextBox ID="TextBox1" runat="server" ></asp:TextBox>

        <asp:Button ID="Button4" runat="server" OnClick="Button1_Click" />

    </form>
</body>

RELEVANT CODES:

protected void Page_Load(object sender, EventArgs e)
{
    TextBox1.Attributes.Add("onkeypress", "return controlEnter('" + Button1.ClientID + "', event)");
}

protected void Button1_Click(object sender, EventArgs e)
{
    Response.Write("Button was Clicked!");
}

That's it... Hope this atricle helps you!