There are a few ways to redirect users to the Service Portal.

The easiest imo is to configure or create a new glide property and add the proper script include to it:

The SPEntryPage script include will evaluate whether to redirect the user to the Service Portal or not:

This tuto explains it quite well:

There are of course other ways to set up your Service Portal for redirection SUCCESS!

Please refer to this excellent community article for more info!