If you include the 'runat="server"' attribute in your input tag, then you should be able to access it from the code-behind. For example:
<input id="inputID" class="textfield" name="myData" maxLength="5" type="text" autocomplete="off" runat="server"/>
can be accessed as such:
inputID.Value = "New Text";
Another StackOverflow article explains this: Accessing HTML Elements in ASP.Net.