On Coinbase Wallet (Desktop) you can switch to Fantom testnet with below steps:
Open the settings from the top right
Enable the 'Developer Mode' checkbox
In settings under 'Default Network' you will find Fantom Testnet
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For Coinbase Wallet (Mobile):
Open Settings in the bottom right
Open Advanced Settings
Switch Network from Mainnet to Fantom Testnet
Go back to settings and choose testnet under 'Default Network' setting
Fantom Mainnet integration
For Fantom mainnet integration, instructions are given in the following link.
Developers: Integrate your dApp with Coinbase Wallet
Fantom developers can easily add support for Coinbase Wallet users by integrating the WalletLink SDK, which lets users sign into dApps with Wallet. Link to Coinbase Wallet SDK.