How to verify ERC721 Smart Contract inherited from Openzeppelin

Hi, I deployed opensea smart contract on rinkeby using truffle and verified using truffle-plugin-verify. After that I deployed same contract on matic testnet. When I try to verify same smart contract on matic I am getting api key error for matic. So tell me how to get that api key from matic. Or if I try to use truffle-flattener then I get this error given below.

Accessing non-existent property ‘INVALID_ALT_NUMBER’ of module exports inside circular dependency on SafeMath File.

See the readme for truffle-plugin-verify.

It worked for me thank you for sharing this repo.