How to use LIKE operator in SQL Query containing Parameter (Variable) in SQL Server

Refer this code

<div>
    <asp:TextBox ID="txtName" runat="server" />
    <br />
    <asp:Button Text="Get employees" OnClick="GetEmployeesByName" runat="server" />
    <asp:GridView ID="GridView1" runat="server" />
</div>

Namespace

using System.Configuration;
using System.Data.SqlClient;

C#

protected void GetEmployeesByName(object sender, EventArgs e)
{
    string constr = ConfigurationManager.ConnectionStrings["conString"].ConnectionString;
    using (SqlConnection conn = new SqlConnection(constr))
    {
        using (SqlCommand cmd = new SqlCommand("SELECT ContactName , CompanyName, City from Customers WHERE ContactName LIKE @SearchTerm + '%'", conn))
        {
            cmd.Parameters.AddWithValue("@SearchTerm ", this.txtName.Text.Trim());
            conn.Open();
            this.GridView1.DataSource = cmd.ExecuteReader();
            this.GridView1.DataBind();
            conn.Close();
        }
    }
}