Where can I find private key for Firebase Cloud Messaging

Under “Cloud Messaging” tab on Firebase, I clicked “generate key pair” and I saw a key pair showing public key, however, when i click on the option “show private key” beside the public key, pop up is empty, may i know what i did it wrong? How do i find the private key generated?

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I’m currently using private key generated on “service accounts” tab, but unsure if it’s the correct way to do it