Put Apple's domain-verification file here, named exactly:

    apple-developer-merchantid-domain-association

(no extension). You get it from your Apple Developer account when you register the
domain against your Apple Pay merchant identifier. server.js serves it at
/.well-known/apple-developer-merchantid-domain-association as text/plain.

Serve it byte for byte — do not reformat it, and do not add a trailing newline that
was not in Apple's file.

If your infrastructure already handles /.well-known, serve it there instead. It does
not have to come from this app; it only has to be reachable over HTTPS at that exact
path on the domain you registered.