(1) In order to use SSL on a root domain (https://acme.com), you'll need a DNS provider which offers the ability to point root domains to a CNAME record. For example Cloudflare, DNS Simple or DNS Made Easy.
(2) If your DNS provider does not support CNAME flattening, you can still use SSL, but your SSL site must be served on the WWW domain (ie: www.acme.com). Use a CNAME to point your WWW subdomain to proxy-ssl.webflow.com and mark it as the default domain in the Webflow Dashboard. Please see Step 3 below to update the A Records for your root domain to re-direct appropriately.
Add the first A Record:
Type: AHost: @Points to: 18.104.22.168TTL: 300
Add the second A Record:
Type: AHost: @Points to: 22.214.171.124TTL: 300
Set your "www" version of your domain as the default domain if your DNS provider does not allow pointing a CNAME or ALIAS record to your root domain.