Required field validator is not working - asp.net

Question

I have used a required field validator, on click of add Button error message will be displayed but whatever code written on onclick event will be executed even if the field is empty.
<div class="form-group posrel">
<asp:Label ID="Label1" runat="server" AssociatedControlID="txtDept"><i class="fa fa-pencil"></i></asp:Label>
<asp:TextBox runat="server" ID="txtDept" placeholder="Department Name" ValidationGroup="ss1"></asp:TextBox>
<div class="text-right validators">
<asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server"
ErrorMessage="Department Name" ControlToValidate="txtDept" ValidationGroup="ss1">
</asp:RequiredFieldValidator>
</div>
</div>
Below is the code snippet of add button -
<div class="form-group pull-right">
<asp:LinkButton runat="server" ID="lnkbtnaddept" CssClass="btn btn-primary" ValidationGroup="ss1" OnClick="lnkbtnaddept_Click">
<asp:Label runat="server" Text="Add" ID="lbladddept"></asp:Label>
<i style="margin-left: 10px;" class="fa fa-send"></i>
</asp:LinkButton>
</div>

Answer 1

May be this will help you out:
On link-button click event validate your page.
Page.Validate("validation group");
if(Page.isValid){
// your code logic
}
for more on page validate follow the link
https://msdn.microsoft.com/en-us/library/0ke7bxeh%28v=vs.110%29.aspx

Answer 2

Turn on JS support for the Validator control by adding attribute:
EnableClientScript="True"
It should prevent a post-back until text box is filled.

Answer 3

take your Id of RequiredFieldValidator and update on back end
or .cs page.
Ans RequiredCityName.Update();