Buy and install a ring lock (like 99 percent of Dutch bikes have) and buy an additional cable\chain that attaches to that ring lock. I should add that insurance for the bikes in Holland depend on the bike having an approved ring lock.

The ring lock is permanently attached to the bike and the cable\chain that attaches to that ring lock is mounted somewhere on the bike. Whether of not you use the chain, the chain is inserted in that ring lock and cannot be taken from the bike.

The key for the lock stays in the lock while you ride and is only removed when you decide to lock it up.

The AXA ring lock here on Amazon US is pricey compared to what I would pay here. Here's a link for the AXA chain for the AXA ring lock. That Amazon link for the ring lock doesn't show the price which is $71. You first have to go to here and scroll down to see the AXA ring lock that accepts a cable\chain.