If you buy a qualifying product directly from DJI, you’ll be offered Care Refresh as part of the checkout process. If you do it is automatically activated. If you buy from another retailer, then you can activate when you first turn it on.
It is possible, though, to join the scheme beyond 48-hours by sending DJI a screen video showing your drone in working operation. The company has set out instructions for creating an Aircraft Verification Video, in which the serial numbers and gimbal calibration must appear.