First and foremost you need to set the CORS configuration for your bucket. I'm a Java developer so here's the code I wrote using the AWS Java SDK v2 to define the CORS configuration for my R2 bucket. Alternatively you can also edit your bucket CORS configuration using Postman following the tutorial at https://kian.org.uk/configuring-cors-on-cloudflare-r2/
// update the config section here with your information first
String bucketName = "YOUR_BUCKET";
String accessKeyId = "YOUR_R2_ACCESS_KEY_ID";
String secretAccessKey = "YOUR_R2_SECRET_ACCESS_KEY";
String accountId = "YOUR_ACCOUNT_ID";
URI uri = new URI("https://" + accountId + ".r2.cloudflarestorage.com");