C# GetElementById

I'm trying to fill in all the fields for League of legends sign-up page (for educational purposes).

Using the code below It should populate the 'date of birth' drop down menu fields, I believe it should work, I've even checked the source code for old programs that others have made where the code is the same yet mine doesn't seem to work. Any idea what I'm doing wrong?

Code:

 private void btnFill_Click(object sender, EventArgs e)
    {

         webBrowser1.Document.All["data[PvpnetAccount][date_of_birth_month]"].SetAttribute("value", "1");

}

Html:

<select name="data[PvpnetAccount][date_of_birth_month]" tabindex="-1" class="ignore_keyup c-signup__input c-signup__input--select c-form-group__input c-form-group__input--select js-selectize selectized" id="PvpnetAccountDateOfBirthMonth" style="display: none;"><option selected="selected" value=""></option></select>

<option selected="selected" value="01">January</option>