How to reattach the EarCup to the headband?

How to reattach the EarCup to the headband?
While I was attempting to fold the headphones, the left earcup broke off and detached from the headband. I noticed the other earcup has a small pin holding it in place, but I seem to have lost the one for the left earcup. What is this pin called, and is it possible to replace it/fix it?

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