Turn your TomTom on, and scroll through the menu screens, by pressing on the forward arrow

until you find the menu option called Itinerary Planning

If you are unable to find Itinerary Planning as an option in this way, then effectively it is not available on your TomTom. If it is an older model, then the functionality was probably never there. If it is a newer model, then it may be there, but “hidden” from view. There are various support forums on the internet which list the models where this might apply, and provide instructions on how to “hack” your TomTom’s menu screen to reinstate the Itinerary Planning menu option so that you can then access that “hidden” functionality.
If you believe your TomTom falls into this latter category, do take a back up first (can be done through TomTom Home), and be aware that if it is still under warranty, any action like this on your part will void that.
If you’re looking to be a new/refurbed TomTom, then if you’re going to a shop, ask for a demo of the model you are interested in. You know what to look for. If buying over the internet, and you have the ability to contact the seller, then you can copy the illustration above into an email to ensure you get a definitive answer on whether your intended purchase has Itinerary Planning. Or, help you get a refund if you are “misled”, as your question will have been hard to misinterpret.