Unable to perform any actions in modal window

I am trying to automate my application and i am unable to perform any actions on the modal window.

Below is the HTML code of that modal window.

<div class="modal fade bs-modal-lg in" id="homeModal" tabindex="-1" style="display: block; padding-left: 16px;" aria-hidden="false"><div class="modal-backdrop fade in" style="height: 958px;"></div>
    <div class="modal-dialog modal-lg">
        <div class="modal-content">
            <div class="modal-header modal-header-dl " id="homeModalHeader">
                <a href="javascript:void(0)" onclick="ClosepopupModal()" class="pull-right dl-padding-top-10 dl-padding-right-20" role="button" title="Close">
                    <i class="zbox-button close"></i>
                </a>
            <h4 class="modal-title">XDS Authentication</h4></div>
            <div class="modal-body " id="homeModalBody"><div class="form-group dl-padding-bottom-20 dl-mg-left-right-5px">
    <input type="text" class="form-control" id="id-number" name="IDNumber" placeholder="SA ID number" value="" maxlength="13" onpaste="return onPaste(event)" onkeydown="return onKeyPress(event)" onkeyup="return onKeyUp(event)" onkeypress="return checkNumberAndClearTextBox('account-number')" onblur="checkLength(this, 13)">
    <small><span id="id-number-error" class="error-msg">&nbsp;</span></small>
    <div>
        <button type="button" id="register-btn" class="btn pull-right dl-button-primary dl-margin-right-0" onclick="GetClientDetails(event)">Retrieve Authentication Questions</button>
    </div>
</div>

I have to enter value on text box and click on a button in that modal window but unable to do it.

Window handles method doesn't worked.Getting unable to locate element error.

driver.SwitchTo().Window(driver.WindowHandles[1]);
            IWebElement IDnumber = driver.FindElement(By.Id("id-number"));
            IDnumber.SendKeys("123456");
            IWebElement Authenticate = driver.FindElement(By.CssSelector(".btn.pull-right.dl-button-primary.dl-margin-right-0"));
            Authenticate.Click();
            IWebElement Skipbutton = driver.FindElement(By.CssSelector(".btn.pull-right.dl-button-secondary"));
            Skipbutton.Click();

What is the right way to do it?