Please refer this code
HTML
<asp:CheckBox ID="chkUPT" Text="UPT" OnCheckedChanged="OnCheckedChanged" AutoPostBack="true"
runat="server" />
<asp:CheckBox ID="chkBLU" Text="BLU" OnCheckedChanged="OnCheckedChanged"
AutoPostBack="true" runat="server" />
<br />
<asp:TextBox ID="TextBox1" runat="server" />
C#
protected void OnCheckedChanged(object sender, EventArgs e)
{
CheckBox checkbox = sender as CheckBox;
if (checkbox.Checked)
{
if (this.TextBox1.Text.Contains(",") || !string.IsNullOrEmpty(this.TextBox1.Text.Trim()))
{
this.TextBox1.Text += checkbox.Text + ",";
}
else
{
this.TextBox1.Text = checkbox.Text + ",";
}
}
}