Feeding WIFI SSID, Passwords and Blynk key’s via a webpage in the ESP8266 EEPROM - From https://community.blynk.cc/t/feeding-wifi-ssid-passwords-and-blynk-keys-via-a-webpage-in-the-esp8266-eeprom/3019/8

esp8266-wifi-param-example.ino

#include <FS.h>                   //this needs to be first, or it all crashes and burns...
//#define BLYNK_DEBUG           // Comment this out to disable debug and save space
#define BLYNK_PRINT Serial    // Comment this out to disable prints and save space
#include <ESP8266WiFi.h>
#include <DNSServer.h>
#include <ESP8266WebServer.h>
#include <WiFiManager.h>          //https://github.com/tzapu/WiFiManager

//for LED status
#include <Ticker.h>
Ticker ticker;

#include <ArduinoJson.h>          //https://github.com/bblanchon/ArduinoJson

char blynk_token[34] = "BLYNK_TOKEN";

bool shouldSaveConfig = false; //flag for saving data

#include <BlynkSimpleEsp8266.h>
#include <SimpleTimer.h>
SimpleTimer timer;

void tick()
{
  //toggle state
  int state = digitalRead(BUILTIN_LED);  // get the current state of GPIO1 pin
  digitalWrite(BUILTIN_LED, !state);     // set pin to the opposite state
}

void saveConfigCallback () {  //callback notifying us of the need to save config
  Serial.println("Should save config");
  shouldSaveConfig = true;
  ticker.attach(0.2, tick);  // led toggle faster
}

void setup()
{

  Serial.begin(115200);
  Serial.println();

  //set led pin as output
  pinMode(BUILTIN_LED, OUTPUT);
  // start ticker with 0.5 because we start in AP mode and try to connect
  ticker.attach(0.6, tick);

  //SPIFFS.format();    //clean FS, for testing
  Serial.println("Mounting FS...");    //read configuration from FS json

  if (SPIFFS.begin()) {
    Serial.println("Mounted file system");
    if (SPIFFS.exists("/config.json")) {
      //file exists, reading and loading
      Serial.println("Reading config file");
      File configFile = SPIFFS.open("/config.json", "r");
      if (configFile) {
        Serial.println("Opened config file");
        size_t size = configFile.size();
        // Allocate a buffer to store contents of the file.
        std::unique_ptr<char[]> buf(new char[size]);

        configFile.readBytes(buf.get(), size);
        DynamicJsonBuffer jsonBuffer;
        JsonObject& json = jsonBuffer.parseObject(buf.get());
        json.printTo(Serial);
        if (json.success()) {
          Serial.println("\nparsed json");

          strcpy(blynk_token, json["blynk_token"]);

        } else {
          Serial.println("Failed to load json config");
        }
      }
    }
  } else {
    Serial.println("Failed to mount FS");
  }
  //end read

  // The extra parameters to be configured (can be either global or just in the setup)
  // After connecting, parameter.getValue() will get you the configured value
  // id/name placeholder/prompt default length
  WiFiManagerParameter custom_blynk_token("blynk", "blynk token", blynk_token, 33);   // was 32 length
  
  Serial.println(blynk_token);

  //WiFiManager
  //Local intialization. Once its business is done, there is no need to keep it around
  WiFiManager wifiManager;

  wifiManager.setSaveConfigCallback(saveConfigCallback);   //set config save notify callback

  //set static ip
  // this is for connecting to Office router not GargoyleTest but it can be changed in AP mode at 192.168.4.1
  //wifiManager.setSTAStaticIPConfig(IPAddress(192,168,10,111), IPAddress(192,168,10,90), IPAddress(255,255,255,0));
  
  wifiManager.addParameter(&custom_blynk_token);   //add all your parameters here

  //wifiManager.resetSettings();  //reset settings - for testing

  //set minimu quality of signal so it ignores AP's under that quality
  //defaults to 8%
  //wifiManager.setMinimumSignalQuality();
  
  //sets timeout until configuration portal gets turned off
  //useful to make it all retry or go to sleep, in seconds
  wifiManager.setTimeout(600);   // 10 minutes to enter data and then Wemos resets to try again.

  //fetches ssid and pass and tries to connect, if it does not connect it starts an access point with the specified name
  //and goes into a blocking loop awaiting configuration
  if (!wifiManager.autoConnect("CentralHeatingAP", "MY123PWD")) {
    Serial.println("Failed to connect and hit timeout");
    delay(3000);
    //reset and try again, or maybe put it to deep sleep
    ESP.reset();
    delay(5000);
  }
  Serial.println("Connected Central Heating System :)");   //if you get here you have connected to the WiFi
  ticker.detach();
  //turn LED off
  digitalWrite(BUILTIN_LED, HIGH);

  strcpy(blynk_token, custom_blynk_token.getValue());    //read updated parameters

  if (shouldSaveConfig) {      //save the custom parameters to FS
    Serial.println("saving config");
    DynamicJsonBuffer jsonBuffer;
    JsonObject& json = jsonBuffer.createObject();
    json["blynk_token"] = blynk_token;

    File configFile = SPIFFS.open("/config.json", "w");
    if (!configFile) {
      Serial.println("Failed to open config file for writing");
    }

    json.printTo(Serial);
    json.printTo(configFile);
    configFile.close();
    //end save
  }

  Serial.println("local ip");
  Serial.println(WiFi.localIP());
  
  Blynk.config(blynk_token);
  Blynk.connect();

}

  void loop()
{
  Blynk.run(); // Initiates Blynk
  timer.run(); // Initiates SimpleTimer  
}