diff --git a/app/Http/Controllers/Api/ApplicationsController.php b/app/Http/Controllers/Api/ApplicationsController.php
index 3dac22efb3..1286f34282 100644
--- a/app/Http/Controllers/Api/ApplicationsController.php
+++ b/app/Http/Controllers/Api/ApplicationsController.php
@@ -239,6 +239,11 @@ class ApplicationsController extends Controller
         }
     }
 
+    private function extraApplicationFieldMessage(string $field): string
+    {
+        return $field === 'fqdn' ? 'Use the domains field to set application URLs.' : 'This field is not allowed.';
+    }
+
     /**
      * Expose sensitive fields on eager-loaded nested Server + ServerSetting
      * relations for callers with the `read:sensitive` or `root` token ability.
@@ -1315,7 +1320,7 @@ class ApplicationsController extends Controller
             $errors = $validator->errors();
             if (! empty($extraFields)) {
                 foreach ($extraFields as $field) {
-                    $errors->add($field, 'This field is not allowed.');
+                    $errors->add($field, $this->extraApplicationFieldMessage($field));
                 }
             }
 
@@ -3394,7 +3399,7 @@ class ApplicationsController extends Controller
             $errors = $validator->errors();
             if (! empty($extraFields)) {
                 foreach ($extraFields as $field) {
-                    $errors->add($field, 'This field is not allowed.');
+                    $errors->add($field, $this->extraApplicationFieldMessage($field));
                 }
             }
 
diff --git a/tests/Feature/ApplicationFqdnFieldApiTest.php b/tests/Feature/ApplicationFqdnFieldApiTest.php
new file mode 100644
index 0000000000..dc7fa07c1f
--- /dev/null
+++ b/tests/Feature/ApplicationFqdnFieldApiTest.php
@@ -0,0 +1,71 @@
+<?php
+
+use App\Models\Application;
+use App\Models\Environment;
+use App\Models\InstanceSettings;
+use App\Models\Project;
+use App\Models\Server;
+use App\Models\StandaloneDocker;
+use App\Models\Team;
+use App\Models\User;
+use Illuminate\Foundation\Testing\RefreshDatabase;
+use Illuminate\Support\Str;
+
+uses(RefreshDatabase::class);
+
+beforeEach(function () {
+    InstanceSettings::unguarded(fn () => InstanceSettings::firstOrCreate(['id' => 0]));
+
+    $this->team = Team::factory()->create();
+    $this->user = User::factory()->create();
+    $this->team->members()->attach($this->user->id, ['role' => 'owner']);
+    session(['currentTeam' => $this->team]);
+
+    $plainTextToken = Str::random(40);
+    $token = $this->user->tokens()->create([
+        'name' => 'fqdn-field-api-test-'.Str::random(6),
+        'token' => hash('sha256', $plainTextToken),
+        'abilities' => ['*'],
+        'team_id' => $this->team->id,
+    ]);
+    $this->bearerToken = $token->getKey().'|'.$plainTextToken;
+
+    $this->server = Server::factory()->create(['team_id' => $this->team->id]);
+    $this->destination = StandaloneDocker::where('server_id', $this->server->id)->first();
+    $this->project = Project::factory()->create(['team_id' => $this->team->id]);
+    $this->environment = Environment::factory()->create(['project_id' => $this->project->id]);
+});
+
+test('PATCH with fqdn points the caller to the domains field', function () {
+    $application = Application::factory()->create([
+        'environment_id' => $this->environment->id,
+        'destination_id' => $this->destination->id,
+        'destination_type' => $this->destination->getMorphClass(),
+        'fqdn' => 'https://app.example.com',
+    ]);
+
+    $this->withToken($this->bearerToken)
+        ->patchJson("/api/v1/applications/{$application->uuid}", [
+            'fqdn' => 'https://new.example.com',
+            'unknown_field' => 'value',
+        ])
+        ->assertUnprocessable()
+        ->assertJsonPath('errors.fqdn', ['Use the domains field to set application URLs.'])
+        ->assertJsonPath('errors.unknown_field', ['This field is not allowed.']);
+
+    expect($application->fresh()->fqdn)->toBe('https://app.example.com');
+});
+
+test('POST create with fqdn points the caller to the domains field', function () {
+    $this->withToken($this->bearerToken)
+        ->postJson('/api/v1/applications/dockerimage', [
+            'project_uuid' => $this->project->uuid,
+            'environment_uuid' => $this->environment->uuid,
+            'server_uuid' => $this->server->uuid,
+            'docker_registry_image_name' => 'nginx',
+            'ports_exposes' => '80',
+            'fqdn' => 'https://new.example.com',
+        ])
+        ->assertUnprocessable()
+        ->assertJsonPath('errors.fqdn', ['Use the domains field to set application URLs.']);
+});