Always seed the demo user with the same API key.

app/database/seeds/UsersTableSeeder.php

@@ -21,6 +21,7 @@ class UsersTableSeeder extends Seeder
                 "password" => "test123",
                 "email"    => "test@test.com",
                 "level"    => 1,
+                "api_key"  => "9yMHsdioQosnyVK4iCVR",
             ],
         ];
 

src/Models/User.php

@@ -65,7 +65,9 @@ class User extends Model implements UserInterface, RemindableInterface
         parent::boot();
 
         self::creating(function ($user) {
-            $user->api_key = self::generateApiKey();
+            if (! $user->api_key) {
+                $user->api_key = self::generateApiKey();
+            }
         });
     }