Shooting for Unsplash+ is by application only. Only those with accepted applications have access to see the briefs, can submit images to Unsplash+, and be eligible for payment.
Once your application is accepted, you will also need to agree to the Unsplash+ artist supply agreement, which covers important details about your relationship with Unsplash and your responsibilities as an Unsplash+ contributor.
You can apply to be an Unsplash+ contributor here.