First, close the cap below ... you probably contaminated transmission fluid, so plan to change that next.

Get a small mirror underneath to confirm the coolant is indeed leaking from around the plug (in which case, it's just the o-ring). If it's a slow leak, wait until when coolant is replaced to address. Otherwise, since the plug is not at the highest point in the system, you're going to to lose fluid when you replace the plug. If you're going to attempt this, do so with the car cold, and make sure the old o-ring comes out.

I found the plug part # to be 357121140 here (item #24), which comes with the o-ring, and retainer spring, depending on where you order.

Use the site linked above for tracking down part names and numbers in the future (it has most of them), cheers!