Hi,
If you enter manually the client ID and client secret in the payment method corresponding settings, it should work automatically.
The client id and secret can be retrieved from your PayPal developer account.
Note also that you can't use the client ID / secret of a live account with the sandbox (so the sandbox setting needs to be turned off). And similarly, you can't use the client ID / secret of a sandbox account with the live environment ( so the sandbox setting needs to be turned on).
Merry Christmas