And I'm stuck on step 3 - Create a directory named.well-knownin the subdomain.
My domain is handled by Google Domains. When contacting Google Support, they responded that 'creating a directory is offered/created by web hosting that is being managed by the web host providers and not registrars like us(Google Domains)'. And advised me to contact the respective web hosting provider - Shopify.
But since it's a third-party domain for Shopify, I cannot manage it here either.