ShrlAlgoToolkit/ShrlAlgoToolkit.RevitAddins/RvFamily/FamilyLibrary/LoadedFamilyDropHandler.cs

using Autodesk.Revit.DB;
using Autodesk.Revit.UI;

namespace ShrlAlgoToolkit.RevitAddins.RvFamily.FamilyLibrary
{
    public class LoadedFamilyDropHandler : IDropHandler
    {
        public void Execute(UIDocument document, object data)
        {
            ElementId familySymbolId = (ElementId)data;
            if (document.Document.GetElement(familySymbolId) is FamilySymbol symbol)
            {
                document.PromptForFamilyInstancePlacement(symbol);
            }
        }
    }
}