The Challenge "#Forzathon To the Finish Line" consists of three goals:
1. Drift 15.000 points in a Porsche.
2. Complete three races in a Porsche.
3. Win a race in a Porsche on Nürburgring GP.
Since goals no. 2 and 3 are pretty straightforward (I used a random Porsche in Free Play), one lap per race, the first goal is kind of a grind.
My impression is that the 15.000 points have to be achieved in one drift, so in order to do this, I made the following modifications:
Firstly, I took a fast Porsche (like the RS 4 or the GT 2 RS) and downloaded a drift tune from the library, just search for the keyword 'drift', you'll find a couple of them. I took one which gave me a 4x4 drivetrain. It's easier to drift with all wheels than just the two in the back, trust me.
Secondly, you need to deactivate all driving assists (especially ESP and TCS, because they will make a drift car undrivable!) and get used to the controls. I switched to cockpit view so I could see my steering wheel, and also the manual shifting helped so the car wouldn't shift up by its own.
Thirdly, repeat until you have got the correct speed and the correct gear (mine worked fine in 3rd gear with about a 140 km/h). Also enable the drift meter in the HUD options so you will see when your car goes into drift.
Why I think that you will need it in one go, is because you can select all different modes for drifting, but it's not directly told which one you must select.
I started a Free Play game with the Race setup "Drift", and didn't change the timer, so you should have 8:00 minutes standardly. In this mode it will count your drift score. As the race track, I chose 'Maple Valley (short)', because it has a very long right hand bend right after start/finish.
Enter this corner at the correct speed and gear (3rd gear @ 140 km/h approximately) and try to stay in the middle of the track. At the end of the turn, you can throw the weight around to the left side. Although you break your drift, the game lets you continue for about 1 to 2 seconds. Just take the next left turn without sliding to the grass and you may get the 15.000 points in one go.
I got this after a couple of turns when I got used to the car's behaviour. (it will also record a small clip if you have this feature installed, just uploaded the clip)
Sorry for the long text. ;)