[NOTE] I'd say this way may not correct. However if you have same trouble in a same situation.
I hope this article might be helpful for you.

Overall

I'd love to use an external authorization which is an API we've already have, however User Auth on Laravel is awesome. Everybody know it.
We must not reinventing the wheel.

So I've summarized that I use external authorization at first, then use User Auth which Laravel has as user filter.

Environment

php 7.1.27
Laravel 5.8.12

Just in case

If you are running MySQL under v5.7.7, add following code to AppServiceProvider.php

app/Providers/AppServiceProvider.php

use Illuminate\Support\Facades\Schema;

...

function boot()
{
    Schema::defaultStringLength(191);
}

Preparation

Run it anyway.

$ php artisan make:auth

In this case, I used same password on Laravel Auth to use an external authorization.
Because of it, we could use Laravel Auth as user filter.

.env

AUTH_PASSWORD=test

config/auth.php

...
    'myauth_password' => env('AUTH_PASSWORD'),
];

Then u can call it like this.

some_file.php

$my_auth_pw = config('auth.myauth_password'); // test

Migration file

$ vim database/migrations/2014_10_12_000000_create_users_table.php

I don't use email column any more.

database/migrations/2014_10_12_000000_create_users_table.php

    public function up()
    {
        Schema::create('users', function (Blueprint $table) {
            $table->bigIncrements('id');
            $table->string('password');
            $table->rememberToken();
            $table->timestamps();
        });
           
        DB::table('users')->insert([
            'id' => 1,
            'password' => bcrypt(config('auth.auth_password'))
        ]);
    }

app/User.php

...
    protected $fillable = [                                                        
        'id', 'password' // change
    ];  
                
...
    protected $casts = [                              
        //'email_verified_at' => 'datetime', // change
    ];

router/web.php

Auth::routes();
Route::post('/login', 'Auth\LoginController@authenticate');

At first, call an external authorization.
Then pass return value to authenticate on Laravel.

app/Http/Controller/Auth/LoginController.php


    private function externalAuth($mail_address, $password)
    {
        ....
        return $id;
    }

    public function authenticate(Request $request)
    {
        $id = $this->externalAuth($request->email, $request->password);
        if (Auth::validate(['id' => $id, 'password' => config('auth.myauth_password')]))
        {
            $user = User::where('id', $id)->first(); // can also use find method
            if($user){
                Auth::loginUsingId($user->id);
                return redirect('/');
            }else{
                return redirect()->back();
            }
        }
        return redirect()->back();
    }

Migrate

$ php artisan migrate

Finally

You can use your email and password for external authorization you've already have,
and you can log in as auth user on Laravel!

Refs

Authorization
Laravel5.4でAuth::attemptを使わずに認証処理をカスタマイズ