Writing a little module for this is also considerable, construct that url through coding is very easy.

I think you need a liitle module which can display a hyperlink like: https://offsiteapp.com/login.aspx?customid=1234

At first, you should know how to create a module, you can use any template to do this, such as template at www.bitethebullet.co.uk

After create the empty module us template, double click the view.ascx file and drag a hyperlinke control on it, named such as "hyperCustomID"

In View,ascx.vb, PageLoad() writh such code:

Dim sConn As String = "Data Source=mydatasource;Initial Catalog=db;UserId=db_user;Password=password;" '
        Dim oConn As New SqlClient.SqlConnection
        Dim oCmd As New SqlClient.SqlCommand
        Dim oReader As SqlClient.SqlDataReader

        oConn.ConnectionString = sConn
        oConn.Open()
        oCmd.Connection = oConn
        oCmd.CommandText = "SELECT customid FROM Users WHERE UserID =" & UserID.ToString()
        'Module base calss provide UserID as a properity
        oCmd.Parameters.AddWithValue("customid", UserID)
        hyperCustomID.NavigateUrl = "https://offsiteapp.com/login.aspx?customid=" & oCmd.ExecuteScalar
        oConn.Close()

BTW: You'd better use DAL of DNN to do the database access .