Using this code
onkeypress="return WebForm_FireDefaultButton(event, '<%=ButtonName.ClientID %>')"
Example
<div style="float: left" onkeypress="return WebForm_FireDefaultButton(event, '<%=btnCheckPin.ClientID %>')">
<asp:TextBox ID="txtPincode" runat="server" Style="padding: 5px;"></asp:TextBox>
<input type="button" class="common_button" onclick="checkDelivery();" value="Check" ID="btnCheckPin" runat="server" />
</div>