on your page load event add this code
this.TextBox1.Attributes.Add("onkeypress", "button_click(this,'" + this.Button1.ClientID + "')");
and write a javascript code like below in script
<script>
function button_click(objTextBox,objBtnID)
{
if(window.event.keyCode==13)
{
document.getElementById(objBtnID).focus();
document.getElementById(objBtnID).click();
}
}
</script>