Quick answer: every hosting plan includes a free SSL certificate for your domains, issued automatically. It is issued once your domain points to your hosting (nameservers or A record) and DNS has updated. Then open your site with https://. If it still shows "Not Secure" after 24 hours, open a ticket.
Steps
- Add the domain to your website: Websites → your site → Domains → Add domain.
- Point the domain to your hosting (see "How to Change Your Nameservers").
- Wait for DNS to update, usually 1 to 2 hours and sometimes up to 24 hours.
- Open
https://yourdomain.com. You should see the padlock.
After it works
- Make your site use https everywhere. In WordPress, set both addresses in Settings → General to start with https.
- If images or styles still load over http, see "Why is my SSL Certificate not working?".