HubSpot doesn't host your root domain(domain.com) and can only host subdomains such as www.domain.com or info.domain.com via CName record.
There are two ways to redirect the root domain to www subdomain:
1. If you'll be hosting your entire website with HubSpot and your DNS provider supports forwarding, set up a redirect in your DNS settings so your non-www domain (such as domain.com) will forward to your www domain (such as www.domain.com).
2. If your DNS provider doesn't support forwarding, you need to set up a basic website hosting and point the A record of your root domain(domain.com) to the IP Address of your web host and set up the redirect in CPanel or via htaccess.
Here are some HubSpot articles that my help you understand the technical requirements and how you can fully go-live with HubSpot: