Web Forms :: FindControl("id"); Does Not Work - Finding Another Alternative?

I have a problem with selecting a control from codebehind.

This is my scenario:

1. i get a string from a webservice (session): string boxstring = "<input type="text" id="MainContent_TextBoxStopRoadName01" name="ctl00$MainContent$TextBoxStopRoadName01">";

2. i insert this string to a <div id="boxcontainer"> from codebehind like this:

boxcontainer.InnerHtml=boxstring;

3. Page loads

4. User types text in the box, and clicks a button to submit it

5. How do i find this textfield from codebehind from its id? I would like to get the field as an TextBox control, but the text value is the most important.

I´ve tried several things, like Findcontrol("id") and Findcontrol("id").Findcontrol("id") and so on. But it doesent work.

Another thing does work though, if instead of a string I create the textfield as an TextBox control, and then insert it: BoxContainer.Controls.Add(TextBox); - then it works! But it is not a possible solution in my situation. I only have the textbox (among many other elements) in a string.