Using Hardhat Etherscan plugin I was able to verify
$ npx hardhat verify --network mainnet 0x8f8350fd36b1da9cc1e92744c8d8186ae0215d3c
Nothing to compile
Successfully submitted source code for contract
contracts/NNNToken.sol:NNNToken at 0x8f8350fd36b1da9cc1e92744c8d8186ae0215d3c
for verification on etherscan. Waiting for verification result...
Successfully verified contract on etherscan
On Etherscan, I then marked your proxy as a proxy, and as the implementation contract was now verified, you can now interact with your upgradeable contract via the proxy on Etherscan.