Nintendo Switch Manual page 29

Push up the slide lock on the bottom of the strap
accessory to lock it in place after you attach it to
the Joy-Con. Then, put on the wrist strap and
push the button on the strap lock while adjusting
the cord so that it doesn't slip from your hand.
How to detach
Push down on the slide lock to unlock the
accessory, then grip the top of the accessory and
slide it up.
Click!