PostcodeAPI.php

<?php
$args = [
      'headers' => [
        'X-Api-Key' => 'Ek7o0F5OX29MzDjQYuDzy9FimBEOKwuukixRryN4'
      ]
 ];
$response = wp_remote_get( "https://sandbox.postcodeapi.nu/v3/lookup/6545CA/29", $args);
 
if ( is_array( $response ) && ! is_wp_error( $response ) ) {
    $body    = $response['body']; # RESPONSE FROM API
    var_dump($body);
}