https://rpc.xdaichain.com/

Follow the steps found here to add a custom RPC network:

https://metamask.zendesk.com/hc/en-us/articles/360043227612-How-to-add-a-custom-Network-RPC-and-or-Block-Explorer

Then follow the steps found here for the custom network settings:

https://www.xdaichain.com/for-users/wallets/metamask/metamask-setup

Specifically:

Network Name: xDai

New RPC URL: https://rpc.xdaichain.com/

Chain ID: 0x64 (100)

Symbol: xDai

Block Explorer URL: https://blockscout.com/poa/xdai