maybe you can find some code snippet in the standard signin form of DNN, maybe similiar code as following:
Private Sub cmdRegister_Click(ByVal sender As Object, ByVal e As EventArgs) Handles cmdRegister.Click
If PortalSettings.UserRegistration <> PortalRegistrationType.NoRegistration Then
If PortalSettings.UserTabId <> -1 Then
' user defined tab
Response.Redirect(NavigateURL(PortalSettings.UserTabId, "", "returnurl=" & HttpUtility.UrlEncode(HttpContext.Current.Request.QueryString("returnurl"))), True)
Else
' portal tab
If PortalSettings.HomeTabId <> -1 Then
Response.Redirect(NavigateURL(PortalSettings.HomeTabId, "Register", "returnurl=" & HttpUtility.UrlEncode(HttpContext.Current.Request.QueryString("returnurl"))), True)
Else
Response.Redirect(NavigateURL("Register", "returnurl=" & HttpUtility.UrlEncode(HttpContext.Current.Request.QueryString("returnurl"))), True)
End If
End If
End If
End Sub