I've just installed my Nextbase dash cam today, so I thought I'd add a post. (My Fiesta is a new Mk8 Titanium X 1.0L auto, however the information below should be true for all mk8 Fiestas)
I used the 'fuse tap' (or piggyback) connector with the Micro2 10A fuse number 3 in the passenger compartment fuse box. This is shown in the handbook (common to all mk8 Fiestas I believe) as 'parking assist module'. The original 10A fuse goes in the fuse tap alongside the 5A fuse for the dash cam, and the fuse tap is slotted in where the original fuse came out. The red lead from the fuse tap terminates with the female part of a bullet connector, and the red (positive) lead from the dash cam plugs in here.
I connected the dash cam's black (negative / earth) lead to earth via the mounting bracket for the fuse box. It's a metal bracket, mounted on a metal stud, with a metal nut securing it. I slackened the nut slightly, slid the spade connector on the end of the black lead underneath and retightened it. This provides a sound earth. Just need to carefully peel back the carpet a few inches to expose the stud.
My dashcam comes on when the ignition is on, and goes off when the ignition is turned off (and it stays on whether the engine is running or not). Job done!
(FYI : I originally tried fuse number 24 as it's shown as spare. It's live, but is permanently live, which is not what I wanted. Also the wiring kit for the Nextbase dash cams come with two different fuse taps included ..a 'standard' and a 'mini'. Neither of these fit the Fiesta's passenger compartment fuse box, as this is populated with Micro2 and Micro3 fuses. Nextbase recognise this and offer a Micro2 fuse tap free on request. They were very helpful, I dropped them an email on Wednesday and the correct fuse tap turned up today, Friday.)
Hope the above helps..