No tools it said - what a clever idea I thought.
Not unless you have the fingers of a rock climber , or what is the secret.
One thing I learnt long, long ago , tie a bit of wire a few links back
either side of the link before removing or replacing to take the tension off
the link area.

With these links the only way I could put one end on the pin and then just
preliminary place the other one was to put a bit of a set in that closure
link, so it would slide over the top of the second pin when firmly pushed by
finger.
Then 2 pairs of molegrips, not particularly firmly gripping but enough to be
able to flex the made-bridge piece enough to complete the locked closure.