Autofill textbox w/ variable

I need to autofill a text box with a variable.

here's what I have so far:
<%
object myVar;
myVar = Request.QueryString["ID"];
int myVarInt = Convert.ToInt32(myVar);
myVarInt = myVarInt + 1;
%>

Works fine. Now I need to do something like:
<asp:TextBox ID="IDTextBox" runat="server" Text='<% myVarInt %>' Width="520px"></asp:TextBox>


Thanks ...