Add and set ASP.Net DropDownList on selection of another using C# and VB.Net

I have a dropdown location see markup below.

<asp:DropDownList ID="ddllocation" runat="server" AppendDataBoundItems="true"  AutoPostBack="true" 
    CssClass="form-control pro-edt-select form-control-primary" Width="100%" >
    <asp:ListItem Text="--Select Location--" />
    <asp:ListItem>Branch1</asp:ListItem>

 <asp:LinkButton ID="btlocation" runat="server" CssClass="btn btn-default" OnClick="btlocation_Click" >Call Location</asp:LinkButton>

The dropdown ddllocation when selected it supposed to pull names from database base on location and then display the name on ddlprocessedby but is not working.

<asp:DropDownList ID="ddlprocessedby" runat="server" AppendDataBoundItems="true"
    CssClass="form-control pro-edt-select form-control-primary" Width="100%" >
</asp:DropDownList>

see code below

protected void btlocation_Click(object sender, EventArgs e)
{
    string constr = ConfigurationManager.ConnectionStrings["DB"].ConnectionString;
    using (SqlConnection con = new SqlConnection(constr))
    {
        SqlCommand command = new SqlCommand("SELECT  Name FROM User3 WHERE [Location]='@Branch1'", con);
        command.Parameters.AddWithValue("@Branch1", ddllocation.Text);
        con.Open();
        SqlDataReader sdr = command.ExecuteReader();
        if (sdr.Read())
        {                   
            ddlprocessedby.SelectedItem.Text = sdr["Name"].ToString();
        }
    }
}

In database table i have this below

UserId Name, Location
----------------------
1      Jake  Branch1
2      Mary  Branch2
3      John  Branch1

So the idea is when location dropdown is selected and button pull location is clicked the names in those locations will be displayed on the dropdown ddlprocessedby.