Custom Domain - Landing Pages - Initial setup

Hello,
I hope someone can help - I feel like I’m going round in circles with this, and don’t understand anything!

The “Set Up Custom Domain” guide on Keap says we require a subdomain - it seems to not like www as a subdomain - is it possible to have -
www.mywebsite.com/my-keap-landing-page
or does it need to be -
subdomain.mywebsite.com/my-keap-landing-page