The External Application item can be found from the Revit Addins category of the Visual Basic Project Items. After the wizard has been filled in like:

A new external application (IExternalApplication) will be created and added to the current project accordingly:

#Region "Namespaces"

Imports System.Text
Imports System.Linq
Imports System.Xml
Imports System.Reflection
Imports System.ComponentModel
Imports System.Collections
Imports System.Collections.Generic
Imports System.Windows
Imports System.Windows.Media.Imaging
Imports System.Windows.Forms
Imports System.IO

Imports Autodesk.Revit.ApplicationServices
Imports Autodesk.Revit.Attributes

Imports Autodesk.Revit.DB
Imports Autodesk.Revit.DB.Events
Imports Autodesk.Revit.DB.Architecture
Imports Autodesk.Revit.DB.Structure
Imports Autodesk.Revit.DB.Mechanical
Imports Autodesk.Revit.DB.Electrical
Imports Autodesk.Revit.DB.Plumbing

Imports Autodesk.Revit.UI
Imports Autodesk.Revit.UI.Selection
Imports Autodesk.Revit.UI.Events

Imports Autodesk.Revit.Collections
Imports Autodesk.Revit.Exceptions
Imports Autodesk.Revit.Utility

Imports RvtApplication = Autodesk.Revit.ApplicationServices.Application
Imports RvtDocument = Autodesk.Revit.DB.Document

#End Region

<Transaction(TransactionMode.Manual)> _
<Regeneration(RegenerationOption.Manual)> _
Public Class ExtApp1
    Implements IExternalApplication

#Region "Cached Variables"

    Public Shared _cachedUiCtrApp As UIControlledApplication

#End Region

#Region "IExternalApplication Members"

    Public Function OnStartup(ByVal uiApp As UIControlledApplication) As Result Implements IExternalApplication.OnStartup
        _cachedUiCtrApp = uiApp

        Try
            'TODO: add you code below.

            Return Result.Succeeded
        Catch ex As Exception
            MessageBox.Show(ex.ToString())
            Return Result.Failed
        End Try
    End Function

    Public Function OnShutdown(ByVal uiApp As UIControlledApplication) As Result Implements IExternalApplication.OnShutdown
        Try
            'TODO: add you code below.

            Return Result.Succeeded
        Catch ex As Exception
            MessageBox.Show(ex.ToString())
            Return Result.Failed
        End Try
    End Function

#End Region

#Region "Local Methods"

#End Region

End Class

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