Here's the html code for the page:
<div class="form-group">
    <label for="TextInput"><font color="#7f2b1b">Account</font></label>
    <input name="Username" class="form-control" placeholder="Enter your Account ID." required="" type="text">
</div>
And here's my attempt of doing it in C#:
foreach (HtmlElement o in webBrowser1.Document.GetElementsByTagName("class"))
        {
            HtmlElement yourElement;
            if (o.GetAttribute("name") == "Username")
            {
                yourElement = o;
                yourElement.SetAttribute("value", "FillingText");
                break;
            }
        }
 
    