GO to Metamask Extension and click here
and scroll down select and click on Custom RPC
you can see a new window
fill and save.
Example BSC SMART CHAIN
BSC
Network Name: Smart Chain
New RPC URL: https://bsc-dataseed.binance.org/
Chain ID: 56
Currency Symbol: BNB
Block Explorer URL: https://bscscan.com
Copy and paste
More info visit at Provided by https://docs.binance.org/smart-chain/wallet/metamask.html
Some other chains like
HECO CHAIN
Network Name: HT- Mainnet
New RPC URL: https://http-mainnet.hecochain.com
Chain ID: 128
Currency Symbol: HT
Block Explorer URL: https://hecoinfo.com/
Tomo chain
Network Name: Tomo chain
New RPC URL: https://rpc.tomochain.com
Chain ID: 88
Currency Symbol: TOMO
Block Explorer URL: https://scan.tomochain.com/
Matic Mainnet
Network Name: Matic Mainnet
New RPC URL: https://rpc-mainnet.matic.network
Chain ID: 137
Currency Symbol: MATIC
Block Explorer URL: https://explorer.matic.network/
Avalanche Network
Network Name: Avalanche Network
New RPC URL: https://api.avax.network/ext/bc/C/rpc
Chain ID: 0xa86a
Currency Symbol: AVAX
Block Explorer: URL: https://cchain.explorer.avax.network/
Energi Network
Network Name: Energi Mainnet
New RPC URL: https://nodeapi.energi.network/v1/jsonrpc
Chain ID: 39797
Currency Symbol: NRG
Block Explorer URL: https://explorer.energi.network/
DRK Chain
Network Name: DRK Chain
New RPC URL: https://rpc.draken.exchange
Chain ID: 66
Currency Symbol: DRK
Block Explorer URL: https://explorer.draken.tech
Network Name: FSN- Mainnet
New RPC URL: https://mainnet.anyswap.exchange
Chain ID: 32659
Currency Symbol: FSN
Block Explorer URL: https://fsnex.com
Network Name: Fantom Opera
New RPC URL: https://rpcapi.fantom.network
Chain ID: 250
Currency Symbol: FTM
Block Explorer URL: https://ftmscan.com/
Hoo Smart Chain
Network Name: Hoo Smart Chain Mainnet
New RPC URL: https://http-mainnet.hoosmartchain.com
Chain ID: 70
Currency Symbol: HOO
Block Explorer URL:https://hscscan.com