To add the Binance Smart Chain network to your Metamask wallet, follow these steps:
In your MetaMask, click the menu which is set to “Main Ethereum Network” by default. In the dropdown click “Custom RPC”.
A form will appear, where you need to fill in certain parameters to connect to the Binance Smart Chain network.
Now switch in the menu from Mainnet Ethereum Network to Binance Smart Chain and you will see the default currency change from ETH to BNB.
Parameters might change. Find a comprehensive guide here: https://academy.binance.com/en/articles/connecting-metamask-to-binance-smart-chain