Merge pull request #806 from telerik/dkrastev/m-KB

KB: Validate License Key in Add-in

Author: nade7o

knowledge-base/add-license-key-in-add-in-project.md

@@ -0,0 +1,44 @@
+---
+title: Telerik License Watermark Appears When UI for WinForms Form is Used in Add-In Project
+description: The invalid or missing Telerik license key warning is displayed in an Add-In project, like Excel/Word VST, and others.
+page_title: Telerik License Key Invalid or Missing Message Box and Watermark Displayed in a Add-in/Plugin Project
+type: troubleshooting
+slug: add-license-key-in-add-in-project
+position: 0
+tags: licensing, installation, telerik, licensing, license, key, not, found, addin
+res_type: kb
+---
+
+## Environment
+
+|Product Version|Product|Author|
+|----|----|----|
+|2025.3.812|Telerik UI for WinForms|[Dinko Krastev](https://www.telerik.com/blogs/author/dinko-krastev)|
+
+## Description
+
+The invalid/missing Telerik license key dialog is shown in addin (like Excel VSTO Add-in) projects even when a valid license is installed.
+
+## Solution
+
+This happens because there is no proper main window and application context, which means that the licensing mechanism cannot be evaluated as expected. To resolve this, use the `TelerikLicensing.Register` static method.
+
+Here is a possible solution to resolve the licensing mechanism and remove the Watermark if the license key is valid:
+
+* Ensure that each project library in your Add-in/Plugin Project is referencing the Telerik.Licensing NuGet. The Licensing NuGet is located on the [NuGet.org server](https://www.nuget.org/).
+
+* Ensure that the watermark does not appear in your Add-In/Plugin when run as a standalone application in Visual Studio. 
+
+* Call the TelerikLicensing.Register() method as early as possible, before initializing any Telerik controls. Keep in mind that RadForm is also a control from our suite. You will need to consider placing it as early as possible in your plug-in’s loading process.
+
+````C#
+
+public class TelerikHelper
+{
+	public static void Register() => Telerik.Licensing.TelerikLicensing.Register();
+}
+	
+````
+
+
+