Hi,
Ah yes, if you have a free shipping method, then just make sure that its ordering is lower than the other ones. That will it will be the first one of the list on your website and it will be automatically selected. Note that to test this, you need to clear your user session and cart (log out if you're logged in, delete the cart in the backend, and login again (or use another browser) ).
Also, if your shipping methods are restricted by zone, you need to make sure that your free shipping method is available in the "main tax zone" that you've configured in your HikaShop configuration. Otherwise, if the free shipping method isn't available for the default zone before the user enters his address, the system won't be able to pre select it.