In the MSDN tutorial here: https://msdn.microsoft.com/en-us/library/ms178232.aspx MSDN suggests the following to set focus in vb.net:

Code:
Protected Sub Page_Load(ByVal sender As Object, _
    ByVal e As System.EventArgs)
    TextBox1.Focus()
End Sub
In my TextBox, I have:

Code:
<asp:TextBox ID="user_name" runat="server" CssClass="textbox" Placeholder="Name" required="required"></asp:TextBox>
How may I set focus, please, to TextBox ID="user_name"?

Thank you.