POST with one extra parameter

Isn't this just a controller action parameter? Can you elaborate a bit?

It just becomes part of the URI for the post? (as you have shown) and will be passed to the saveEvent in the method call.

Sure, change your route to this:

$app->post('api/v1/event/{api?}','EventController@saveEvent');

Then the controller...

public function saveEvent(Request $request, $api = NULL) {
  // If included in the request url, $api will be set to that value, else null by default.
  // json is contained within $request->input
}

It works when I access using

/api/v1/event/1234

I get 1234 in $api and the JSON on $request->getContent()

It doesn't work if I send /api/v1/event with an empty api, is possible to use it without api?

Maybe optional params aren't allowed for post routes? Try this...

routes:

$app->post('api/v1/event','EventController@saveEvent');
$app->post('api/v1/event/{api}','EventController@saveEvent');

controller:

public function saveEvent(Request $request, $api = NULL) {
  // If included in the request url, $api will be set to that value, else null by default.
  // json is contained within $request->input
}

It works, thank you very much!