A workaround for this issue is to use hiddenfields. Although the values of Username and Password RadTextBoxes are not passed to server they can be grabbed by JavaScript. With JS, the username and password can be set as a hiddenfield's value and their value will be passed during postback. Do this during OnClientClicked event of Button.
Client Side:
<script>
function setHiddenFields() {
var userNameTxt = $find("<%= UserName.ClientID %>");
var passwordTxt = $find("<%= txtPassword.ClientID %>");
var name = userNameTxt._textBoxElement.value
var pass = passwordTxt._textBoxElement.value
$("#<%=hdnUserName.ClientID %>").val(name);
$("#<%=hdnPassword.ClientID %>").val(pass);
}
</script>
<div id="loginDiv>
<telerik:RadTextBox ID="UserName" runat="server"></telerik:RadTextBox>
<telerik:RadTextBox ID="Password" runat="server" TextMode="Password"></telerik:RadTextBox>
<telerik:RadButton ID="LoginButton" runat="server" Text="Giriş" OnClientClicked="setHiddenFields"
OnClick="LoginButton_Click"> </telerik:RadButton>
<asp:HiddenField ID="hdnUserName" runat="server" />
<asp:HiddenField ID="hdnPassword" runat="server" />
</div>
Server side:
protected void btnGiris_Click(object sender, EventArgs e)
{
string username= UserName.Text;
string pass = Password.Text;
if (string.IsNullOrEmpty(username))
username = hdnUserName.Value;
if (string.IsNullOrEmpty(pass))
pass = hdnPassword.Value;
.
.
.
}