You need to read the docs and links carefully.
If you have your own (selfwritten) app then you can’t use this app to do it.
You either need the Particle app, use one of theSoftAP solutions or have to build the SoftAP logic into your own app (which it obviously has not, otherwise you would not be asking this question ;-)).
And the communication is not BT but WiFi (even in Listening Mode).
The alternative way via your device firmware (by means of
WiFi.setCredentials()) would only be available while you got cloud connection, which seems to be not available given your description.
But if you had your device connected to the cloud (e.g. via your mobile phone AP), you might be able to setup a
Particle.function() to achieve this.