I solved it myself. I changed the code and just added a button and put this code in:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data;
using System.Drawing;
using System.Text;
public partial class FinancialIndicatorsFormA : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{
}
protected void ButtonCalculate_Click(object sender, EventArgs e)
{
int a = Convert.ToInt32(TextBoxTA.Text);
int b = Convert.ToInt32(TextBoxTL.Text);
TextBoxTNA.Text = Convert.ToString(a - b);
}
}