So as the description says this is for equipping a NASCAR platinum badge.
To do this go to options from the main menu then select change badge. Use the
to get to NASCAR badges. This will show you what you need to do to earn the badges.
If you own the dlc you can just equip the one for owning the dlc (can someone confirm or deny this being platinum as I don't own the dlc).Infinitelogic has confirmed that the badge for owning the dlc is not platinum- " Just to confirm that the badge for owning the Nascar DLC is NOT platinum, so I would suggest you go for the Perfect Draft or Distance driven in a Nascar badges if you want this quickly, otherwise just complete the Nascar DLC which will give the Platinum Expedition badge."
If you
do not own the dlc you have 2 options of badge to go for: Perfect drafts and multiplayer quick stop races. My advice is to go for the perfect drafts as the platinum nicely lines up with this achievement for getting 100.
Once you have a platinum unlocked just hit
with it highlighted and the achievement will unlock.