HTTP Link header
Link: <https://example.com/>; rel="alternate"; hreflang="en-US", <https://example.com/uk/>; rel="alternate"; hreflang="en-GB", <https://example.com/es/>; rel="alternate"; hreflang="es-ES", <https://example.com/>; rel="alternate"; hreflang="x-default"
Sitemap (xhtml:link)
<url xmlns:xhtml="http://www.w3.org/1999/xhtml">
<xhtml:link rel="alternate" hreflang="en-US" href="https://example.com/" />
<xhtml:link rel="alternate" hreflang="en-GB" href="https://example.com/uk/" />
<xhtml:link rel="alternate" hreflang="es-ES" href="https://example.com/es/" />
<xhtml:link rel="alternate" hreflang="x-default" href="https://example.com/" />
</url>