Add Polygon / MATIC to MetaMask:
Click the MetaMask icon in your browser.
Click the drop-down that says "Ethereum Mainnet".
Click "Add Network".
Enter the following information and then click Save.
Network Name: Polygon Mainnet
New RPC URL: https://polygon-rpc.com
ChainID: 137
Symbol: MATIC
Block Explorer URL: https://polygonscan.com/
(Source: https://docs.polygon.technology/docs/develop/metamask/config-polygon-on-metamask/