How to Access my HTML <input ..> TextBox in ASP.NET

Question

I want to access my HTML input Text-type text box in my backend c# code. When i does it return NULL value.

<input type="email" runat="server" class="form-control" placeholder="Email address" id="emailBox" />
What i have done yet: 
1.string un = Request.Form["emailBox"];
2.  string unemailBox.Value
All time it, return NULL Value.