Answer
NOTE: this answer only refers to older devices, around 2015 and before.
If you open the preferences of your TomTom device (touch the screen somewhere and tap ‘Edit preferences’), you can see how many preference screens you have in the upper left corner. If it says ‘1 of 3’, you will be unable to find the planning preferences. They are hidden. But do not worry: you can make them appear. Follow these steps:
Go to the third preference page (‘3 of 3’) and look for an option that says ‘Display all menu items’.
Tap that option. Your TomTom device will tell you that all menu items will be available from now on.
Open the preferences again: touch the screen somewhere and tap ‘Edit preferences’.
Now you will have 7 preference screens (‘1 of 7’). Go to page 4. There you will find an option called ‘Planning preferences’. (It comes with an icon that contains a car, a bike and a walking person.)
Now you can choose the ‘Ask me every time I plan’ option.
As far as I know, this does not count for the TomTom ONE 3rd and 4th Edition. Those devices do not have itinerary planning functionality. It is not possible to load, save and use itinerary files therefore on those devices. Unless you manually edit the menu structure of the TomTom ONE 3rd/4th Edition: you may use the TomTom Menu Designer to do that: click here for more information.