You can set your button as DefaultButton putting textbox and button in a panel
<asp:Panel ID="Panel1" runat="server" DefaultButton="Button1">
<asp:TextBox runat="server" ID="txtcity" ></asp:TextBox>
<div id="resultDiv" runat="server>
</div>
<cc1:AutoCompleteExtender ID="txtcity_AutoCompleteExtender" runat="server" MinimumPrefixLength="1"
ShowOnlyCurrentWordInCompletionListItem="true" CompletionSetCount="10" ServiceMethod="getcityName"
ServicePath="webservice.asmx" CompletionInterval="1000" CompletionListElementID="resultDiv"
EnableCaching="true" TargetControlID="txtcity">
</cc1:AutoCompleteExtender>
<asp:Button ID="Button1" runat="server" Text="Button" OnClick="Button1_Click" />
</asp:Panel>
<pre lang="text">