Spin the cranks so that the link is on the top run of chain.
Stand on pedals.
The link will click into place.
This. Though you don’t usually need to stand on the cranks… if the bike is in the stand just put the rear brake on and push on the pedal with your other hand.
You’ll need a pair of pliers to get them apart though!
I’ve only ever had this when opening the link again immediately after installation. After you’ve put a few miles on the chain, the normal squash the plates together and compress the link at the same time by hand trick always works.
I’ve given up and just used a chain tool instead for now
I’ve found the old school chain tool rejoining method to be unreliable on modern (well, 10 speed) chains.