Search engines are always your best friends. I got this one in Google
Enter Key in ASP.NET - Complete Research[
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Sample code
txtUsername.Attributes.Add("onkeydown", "if(event.which || event.keyCode){if ((event.which == 13) || (event.keyCode == 13)) {document.getElementById('" + txtPassword.UniqueID + "').focus();return false;}} else {return true}; ");