Support for resource hints in WordPress is done through rel attribute of <link> elements whithin the HTML docuemnt, not with HTTP/2 server push (which uses Link headers in the HTTP response). You can use wp_resource_hints filter to add the URLs you need the prefetch like this:
add_filter( 'wp_resource_hints', 'cyb_resource_hints', 10, 2 );
function cyb_resource_hints( $urls, $relation_type ) {
if( 'prefetch' == $relation_type ) {
$urls[] = 'https://exmaple.com/assets/script.js';
}
return $urls;
}
If you prefer to use HTTP/2 server push, you can set the Link header with PHP, .htaccess on Apache, etc.
For example, with PHP:
header("Link: </css/styles.css>; rel=preload; as=style");
And you can integrate it with WordPress at multiple levels, usually template_redirect action, for example:
add_action( 'template_redirect', 'cyb_push_styles' );
funciton cyb_push_styles() {
header("Link: </css/styles.css>; rel=preload; as=style");
}