File: Properties\AssemblyInfo.cs
Web Access
Project: src\src\Controls\src\Core.Design\Controls.Core.Design.csproj (Microsoft.Maui.Controls.DesignTools)

using Microsoft.VisualStudio.DesignTools.Extensibility.Metadata;
 
[assembly: ProvideMetadata(typeof(Microsoft.Maui.Controls.Design.RegisterMetadata))]