I am trying to purchase a One Day Travelcard to cover me from Zones 1-6 to Tilbury Town with a 16-25 R/C for tomorrow. How do I enter this into the online booking engines? And should I use a specific site?
The out-boundary travelcard is intended for starting and finishing outside the zones. To make a single journey out of the zones and back in again you'll need to buy a zones 1-6 travelcard and a boundary zone 6 to Tilbury Town return. You can buy the travelcard online, but the boundary ticket can only be bought at a station, or possibly over the phone with the travelcard. If the trip to Tilbury is later on in the day then you can buy the boundary ticket at any time up to starting the journey in question, so whenever you have a spare moment.
However, a Tilbury - Z1-6 Travelcard wouldn't help in that regard even if the OP was travelling in the other direction - as you can only use a Day Travelcard for one journey to/from the London zonal boundary.