Simply spend 200,000 credits on cars in the Autoshow or on upgrades for cars you already own.
If you do not have 200,000 credits, go onto the Forza Hub app where you can get credits under the welcome tab. The amount of credits you receive is based on what tier you are in. The credits will take a short amount of time to appear under the gifts tab in the message center in game.
Alternatively, you could complete bucket lists, races and playground games to get more credits.
Alehldean confirms that you can put a bid on a car of at least 200,000 credits in the Auction House and unlock the challenge. You don't have to win the auction to unlock the challenge.
Barrwg also states that you can unlock the challenge by spending 200,000 credits on the wheelspins.
If there are any improvements you feel could be made to this guide, please state them in the comments.