What is the IP address of my AWS website so I can add the A record on Cloudflare?

I've bought a web domain with AWS Route 53 and set up the S3 buckets required, followed the AWS guide and got my site all working without cloudflare. Then I wanted to get my site to be secured with HTTPS and so I applied for the certificate on AWS Certificate Manager, giving me the CNAME and value which I put into cloudflare.. Now it's all setup, but the site won't resolve. I have the certificates on AWS issued, BUT cloudflare is still saying this:

Add an A, AAAA, or CNAME record for www so that www.mysite.com will resolve.

Add an A, AAAA, or CNAME record for your root domain so that mysite.com will resolve.

I only have the two CNAME records and TXT record on cloudflare. The problem is, I have no idea how to get the IPv4 address for my AWS site, which is required to add an A record on cloudflare:

| Type A | Name (required), Use @ for root | IPv4 address (required) |

I should also add I have done this before somehow on my other basic AWS S3 site and there is an IPv4 address on the cloudflare for that, and I don't think I remember ever finding it or adding it manually. How did it get there? And how can I get this A record IP address on cloudflare for my new site?