1. On an existing S2C connection
Unfortunately, the PFS can't be disabled on an existing S2C connection. The customer would be required to delete and recreate the connection without PFS
2. On a new S2C connection
When you create a S2C connection, select Azure VPN as Remote Gateway Type
In the connection details, the customer would see PFS set to a value defined in the algorithms, but you can ignore that.
If you check the gateway S2C config, you won't see PFS enabled