Custom domains are supported under paid plans.
You can set your custom domain in account settings
You can upgrade to a paid plan in subscriptions
Please note that custom domains unfortunately do not support SSL.
For SSL support, you will need to link to your public SlimFAQ url.
e.g. if you have the a SlimFAQ under https://slimfaq.com/faq and you have the custom domain set to faq.example.com, you will be able to access the FAQ on both, https://slimfaq.com/faq and http://faq.example.com - but https://faq.example.com will not be supported.