DiRT Rally 2.0

Edit: To avoid any confusion, The BMW M1 is DLC. It comes with the Year One Season pass for $19.99 or it is sold as a separate add on for $1.49. Please check which achievements are included with the base game and which are add ons.

Let's start things off by saying the BMW M1 Procar Rally is part of the Group B (RWD) division.
In my opinion the BMW slides out a bit more than the Audi Sport Quattro that you need for the achievement below:

With that being said, you may need a bit of seat time in this car.
Note: Sonic6L in the comments has confirmed you DO NOT need to win, only finish the event. (I can not confirm if a win is needed, however the description makes it sound like any placement will work.)

You can complete this achievement in Career, Historic Rally: Back to the 80s or in Custom Championships.

The quick and easy way would be to create a custom championship as it will only be one race. Once you click "Create Championship" it will ask for a destination. This is personal preference as I do not know what your favorite stage is, however I tend to choose Spain.

Once you choose Spain, it will ask for suface settings and vehicle class. Here you must change the class to Group B (RWD). Once you confirm it will shoot you to stage select/confirm. I personally switched the stage to the far right, a race called "Subida por carretera". It is 100% tarmac and only takes about a 2 min 20 seconds. To make things easier make sure your conditions are set to Daytime/Clear/Dry Surface and equip Medium/Hard tires.

All that is left now is to race and win the championship.

TL;DR
1. Freeplay/Custom Championship
2. Add Event
3. Spain (Or your favorite)
4. Class: Group B (RWD)
5. Change stage to Subida por carretera (Daytime/Clear)
6. Start Championship
7. The BMW is the 3rd car from the left

The race is about 2 min, 20 seconds