To use this tool, you need the following (or equivalent on your photon device's firmware)

// in a header
#if Wiring_WiFi
#include "softap_http.h"
#define SOFTAP_PREFIX  "PHOTON"
System.set(SYSTEM_CONFIG_SOFTAP_PREFIX, SOFTAP_PREFIX);
void app_page_handler(const char* url, ResponseCallback* cb, void* cbArg, Reader* body, Writer* result, void* reserved);
inline void particle_startup() { softap_set_application_page_handler(app_page_handler, nullptr); }  // must get called by your STARTUP() macro somehow
#endif


// in a .cpp
#if Wiring_WiFi
void app_page_handler(const char* url, ResponseCallback* cb, void* cbArg, Reader* body, Writer* result, void* reserved) {
    if (strcmp(url, "/reset") == 0) {
        System.reset();
    }
    // other urls are handled by system handler
}
#endif

For this firmware to work properly, you may need to run your device in SYSTEM_THREAD(ENABLED); mode. You may also need to manually trigger entering listening mode with WiFi.listen() as appropriate for your application.

To run the tool you need to load up the web page locally on your device first, and then connect to the device's hotspot.