In most cases I recommend you let mojoportal handle the redirecting after login unless you have a very specific reason for forcing it to a specific page. It can be a bad user experience to be redirected to an unexpected page after sign in.
By default mojoPortal will redirect to whatever page the user was already on before he clicked the login link. For example if the user is looking at the forums and wants to post he has to login first so he clicks the link and the login page smartly redirects him back to the forum after he signs in.
I implemented a simple feature for this today so it will be in the next release coming later this week, but again I generally don't recommend using this feature.
In the next release you will be able to make a setting to force redirecting to a specific page:
<add key="PageToRedirectToAfterSignIn" value="/yourcustompage.aspx" />
However, this feature will not work for a multi site installation, only for a single site installation. If you need more control than that you can implement a custom SignInHandler.