This assumes that ``notification_url`` ends in a proper slash, could use ``join`` instead to construct new URIs,

```
irb(main):005:0> URI.join("https://ws.sandbox.pagseguro.uol.com.br/v2/transactions/notifications","12345")
=> #<URI::HTTPS:0x007fefe33952f8 URL:https://ws.sandbox.pagseguro.uol.com.br/v2/transactions/12345>
```