SSL certificate for custom branch subdomain

Hi, I’d like to request an SSL certificate extension for a custom subdomain pointing to a branch deploy, following the support guide “How to use Netlify’s branch deploy feature without Netlify DNS.”

Site name: cityclues
Branch: testing
Custom subdomain: testing.cityclues.co.uk
CNAME target: testing–cityclues.netlify.app

I’ve added the CNAME record at my DNS provider (Cloudflare, DNS only / not proxied) and it’s resolving correctly. Could you please extend the SSL certificate for this site to cover testing.cityclues.co.uk or talk me through how I can do it? I’ve done my main production site already, just unable to do my testing branch.

Thanks so much.