Yes, you can use a custom subdomain e.g. mysubdomain.mycustomdomain.com
This could be a good “use case” if you already have a website to promote your brand, but you need something else to support your sales funnel strategy.
To set a custom subdomain you will need to create an A-Record for that subdomain on your domain register (e.g. GoDaddy or NameCheap).
Please to refer to the process for setting up a custom domain via an A-Record.
IMPORTANT: If you plan to Facebook Ads you will need to verify the “parent” domain i.e. you cannot run Facebook Ads on a subdomain like “mysubdomain.mycustomdomain.com” unless you have separately verified “mycustomdomain.com”.