add_action('wp_header', 'mifuncion')

echo '<link rel="manifest" href="ruta-al-plugin/manifest.json">';

echo '<link rel="manifest" href="ruta-al-plugin/manifest.php">';

<?php
$manifest = array(
    "name"=> "Progress Web App",
    "version"=> "0.3",
    "short_name"=> "PWA"
);
header('Content-Type: application/json');
echo json_encode($manifest);
?>