You can’t use a zones 1-2 or zones 1-6 Travelcard to travel to Watford Junction station. Watford Junction is in Zone 9, so you’ll need a separate ticket. You can buy a return train ticket to Watford Junction from National Rail stations, such as Euston.
Alternatively, you can buy a Travelcard covering zone 9 to travel to Watford Junction. A zone 9 Travelcard can be bought from tube station ticket offices in London. You can also travel with a Visitor Oyster card, which can be preloaded with credit to cover your journey to Watford Junction. Visitor Oyster cards are available from the VisitBritain shop and will automatically calculate the correct fare for your journey.