firebase authentication with facebook

hi unity community

i am trying to add firebase authentication into my game, but the documentation in the firebase site is a bit confusing. i found this code on the unity forums:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using Facebook.Unity;
 using Firebase.Auth;
 using UnityEngine.UI;
 
 public class facebookAuthenticator : MonoBehaviour
 {
     public Text userID;
     private FirebaseAuth facebookAuth;
 
     private void Awake()
     {
         if (!FB.IsInitialized)
             FB.Init();
         else
             FB.ActivateApp();
     }
 
     public void logIn()
     {
         FB.LogInWithReadPermissions(callback: onLogIn);
     }
 
     private void onLogIn(ILoginResult result)
     {
         if (FB.IsLoggedIn)
         {
             AccessToken tocken = AccessToken.CurrentAccessToken;
             userID.text = tocken.UserId;
             Credential credential = FacebookAuthProvider.GetCredential(tocken.TokenString);
         }
         else
             Debug.Log("log failed");
     }
 
     public void accessToken(Credential firebaseResult)
     {
         FirebaseAuth auth = FirebaseAuth.DefaultInstance;
 
         if (FB.IsLoggedIn)
             return;
 
         auth.SignInWithCredentialAsync(firebaseResult).ContinueWith(task =>
         {
             if (task.IsCanceled)
             {
                 Debug.LogError("SignInWithCredentialAsync was canceled.");
                 return;
             }
             if (task.IsFaulted)
             {
                 Debug.LogError("SignInWithCredentialAsync encountered an error: " + task.Exception);
                 return;
             }
 
             FirebaseUser newUser = task.Result;
             Debug.LogFormat("User signed in successfully: {0} ({1})",
                 newUser.DisplayName, newUser.UserId);
         });
     }
 
 }

by all accounts on the feed, the code works. however, i do not understand how i would invoke this in my game? how can i get users to fill in their info and pass it to this? do i need a button saying "sign in with facebook" and link it to this script? please help.