Step 1 - Select domain you want SSL activated in HubSpot's Domain Manager
Log into HubSpot and request an SSL certificate for your domain. It'll take a up to 72 hours for them to get back to you with one of their Let's Encrypt "Validate your domain" emails.
Step 2 - Sign up and set up Cloudflare Free plan for your domain
Set up your website onto Cloudflare's Free plan. I have written a guide here if you need it.
Step 3 - Get Source URL and Destination URL
Get the Source URL and Destination URL details from HubSpot's Domain Manager.
Step 4 - Add the redirect to Cloudflare
Add the redirect to Cloudflare in the Page Rules tab.
Select the "Forward URL" option.
302 - Temporary Redirect is sufficient