The easiest way to make your Project available via HTTPS on its custom domain is with Cloudflare, which has a free tier that provides HTTPS.
Here's a quick video explaining how it works:
Cloudflare requires you to change your nameservers to point to them. Meaning that the DNS records are then handled on the Cloudflare side. So when you're setting up the site in Cloudflare, you'll need to make sure those A records are correct.
Once you've got the site all set up in Cloudflare and the DNS changes have propagated (this may take up to one day), you'll be able to view your site in HTTPS.
It's up to you to only use secure resources to keep that "Secure" light green!