bind the text box to another text boxes

string ConStr = @"server="serverName";database=dbName;uid=username;pwd=password";
            DataSet ds = new DataSet();
            SqlConnection Con = new SqlConnection(ConStr);


            Con.Open();

            SqlCommand cmd = new SqlCommand("Select EMPNAME from TableName Where id='"+txtid.text+"'", Con);
            txtEMPNAME.text = Convert.ToString(cmd.ExecuteScalar());
            
               
                Con.Close();

To add to previous answers, never concatenate values directly to your SQL statements. This will leave you vulnerable to SQL injections, data type conversion mismatches and so on.

The proper way to use user input in a SQL statement is to use a SqlParameter[^]

So your code could look like:

...
SqlCommand command = new SqlCommand("SELECT EmpName FROM YourTable WHERE Id = @Id", Con);
command.Parameters.AddWithValue("@Id", TheTextBox.Text);
...

you can do it same without post back of your page by using ajax.

function GetNameById(EmpCode,EmpName)
        {
            

            var retVal = doAjax('frmGetDetail.aspx','EmpCode='+EmpCode.value,0);

            if(retVal.length>0)
            {
                EmpName.value=retval;
                return true;
            }
            else
            {
                alert('The Employee Code Is not Exist.');
                return false;
            }
                

        }


function doAjax(url,query,getxml) 
    {
        var req;
        
        try {
         req = new XMLHttpRequest(); /* e.g. Firefox */
         } catch(err1) {
           try {
           req = new ActiveXObject('Msxml2.XMLHTTP'); /* some versions IE */
           } catch (err2) {
             try {
             req = new ActiveXObject("Microsoft.XMLHTTP"); /* some versions IE */
             } catch (err3) {
              req = false;
             }
           }
         }
        req.open("POST", url + '?' + query,false);
        req.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
        req.send();
        if(req.readyState == 4) 
        {
           if(req.status == 200) 
           {
              var item = req.responseText;
              if(getxml==1) 
              {
                 item = req.responseXML;
              }
           }
        }
        return item;
}

in your .cs page add attribute to your textbox in which you enter id

protected void Page_Load(object sender, EventArgs e)
    {

txtEmpcode.Attributes.Add("onblur", "return GetNameById(" + txtEmpcode.ClientID + "," + txtEmpName.ClientID + ")");
}

now you add a page of ajax in which you call your query to get empname:-
in this aspx page only enter page directive which i have written and no any line needed to add

<%@ Page Language="C#" AutoEventWireup="true" CodeFile="frmGetDetail.aspx.cs" Inherits="frmGetDetail" Theme=""%>

now in this page cs call your query like that

protected void Page_Load(object sender, EventArgs e)
    {
        Response.Write(getEmpNameById(Request.QueryString["EmpCode"].ToString()));
    }

private string getEmpNameById(string EmpCode)
        {
string ConStr = @"server="serverName";database=dbName;uid=username;pwd=password";
            DataSet ds = new DataSet();
            SqlConnection Con = new SqlConnection(ConStr);
 
            Con.Open();
 
            SqlCommand cmd = new SqlCommand("Select EMPNAME from TableName Where id='"+ EmpCode +"'", Con);
            ds= cmd.ExecuteDataSet());
            Con.Close();
               if (ds.Tables[0].Rows.Count > 0)
                  return(ds.Tables[0].Rows["EmpName"].ToString())
               else
                 return ""; 
                
        }