Well, you can't use the signed requests from Laravel in your app since they are signed with a unique formula within the framework. However, if you use the same hashing algorithm in your app you should be able to decode the hash.
'signature' => hash_hmac('sha256', $this->route($name, $parameters, $absolute), $key),
I Laravel the is the $key here the APP_KEY that is defined in your .env
Note that this is really complicated to get this right. I would rather suggest you to use a different kind of hash or are a base64 hash as an example.