In order to accept payment using Apple Pay on your custom built website, you will need to upload a file to a specific location.
Download this domain association file and host it at
/.well-known/apple-developer-merchantid-domain-association on your site. For example, if you're registering
https://example.com, make that file available at
Once this file is publicly available, it can take an hour or two for Apple Pay to register in Apple's system for your domain.
To test Apple Pay is working and enabled, you will need the correct hardware. Here's a guide from Apple.