If an Underground station does not share a connection with a UK rail station it won’t appear as a destination when you’re searching. Instead you’ll need to know what zone the station is in.
Let’s take Canary Wharf as an example. It’s in zone 2. It won’t appear when you search for it as a destination, so you’ll need to populate the destination field with ‘Zone U2356 London’. If going to a station in Zone 1 select Zone U12356 London. The numbers correspond to the zones you need to travel through to get to your destination zone.
Select when you want to travel, and choose rough times to display Peak or Off-Peak options.
Don’t worry about all the times that are given, you’ll be able to travel on any trains with an Anytime Day Return (unless you’re buying an off-peak ticket).