What time are you due at Harry Potter Studio? This info is From their website about travelling from Euston and helps determine how much time you'll have beforehand.
The nearest railway station is Watford Junction; there are direct, fast train services from both London Euston (20 minute journey) and Birmingham New Street (1 hour journey). Other non-direct train services operate from London Euston however these may take longer to arrive at Watford Junction.
A shuttle bus, operated by Mullany's Coaches runs from Watford Junction to the Studio Tour, with a journey time of around 15 minutes. Plan to arrive at Watford Junction 45 minutes before the time stated on your ticket, and have your ticket or booking confirmation ready when boarding the shuttle bus.