WiFiManager/examples/Unique/pubsubclient/pubsubclient.ino

#include <WiFiManager.h>
#include <PubSubClient.h>
#include <ArduinoJson.h>

WiFiClient espClient;
PubSubClient client(espClient);

char mqtt_server[40];

void mqttCallback(char* topic, byte* payload, unsigned int length) {
  // message received
}

void mqttReconnect() {
  // reconnect code from PubSubClient example
}

void setup() {
  Serial.begin(115200);
  WiFiManager wifiManager;
  wifiManager.setTimeout(180);

  if(!wifiManager.autoConnect("AutoConnectAP")) {
    Serial.println("failed to connect and hit timeout");
    delay(3000);
    ESP.restart();
    delay(5000);
  } 

  // id/name, placeholder/prompt, default, length
  WiFiManagerParameter custom_mqtt_server("server", "mqtt server", mqtt_server, 40);
  wifiManager.addParameter(&custom_mqtt_server);

  const char* mqtt_server = custom_mqtt_server.getValue();

  Serial.println("connected...yeey :)");

  client.setServer(mqtt_server, 1883);
  client.setCallback(mqttCallback);
}

void loop() {
  if(!client.connected()) {
    mqttReconnect();
  }
  client.loop();
  yield();
}