<asp:TextBox runat="server" ID="uid" AutoPostBack="True" Text=""></asp:TextBox>

protected void Page_Load(object sender, EventArgs e)
    {
        if(IsPostBack && uid.Text != "" && pw.Text == "")
        {
            Do stuff
        }
    }

<asp:TextBox runat="server" ID="uid" Text="" AutoPostBack="True" OnTextChanged="UidTextChanged"></asp:TextBox>

protected void UidTextChanged(object sender, EventArgs e)
    {
        ClientScript.RegisterStartupScript(GetType(), "alert", "alert('it works');", true);
    }

<asp:TextBox runat="server" ID="uid" AutoPostBack="True" Text="" onblur="__doPostBack('','');" OnTextChanged="UidTextChanged"></asp:TextBox>

<asp:TextBox runat="server" ID="uid" AutoPostBack="True" Text="" onblur="__doPostBack('','');"></asp:TextBox>

"Uncaught TypeError: Property 'submit' of object #<HTMLFormElement> is not a function
__doPostBack
(anonymous function)"

<asp:TextBox runat="server" ID="uid" AutoPostBack="True" Text="" OnTextChanged="TextBox1_TextChanged"></asp:TextBox>

protected void TextBox1_TextChanged(object sender, System.EventArgs e) {
    .....
}