How to create a Sub in VBA that is executed when its correspondent Add-In is loaded?

I have a subroutine in VBA that is executed when we open a PowerPoint presentation but I want to execute that Sub when an Add-in is loaded instead.

How can I do that?

If I'm understanding you correctly, you have an add-in that you want to run the sub-routine of an existing slide deck only when the addin is loaded.

If so, here are instructions on how to do this:

1. Create a slide deck, save it as "Presentation3.pptm" (macro-enabled PPT for 2007). Open the VBE and put in the following code:

   Sub AddText()
   Dim p As Presentation
   Set p = ActivePresentation
   Dim sh As Shape
   Set sh = p.Slides(1).Shapes.AddTextbox(msoTextOrientationHorizontal, 100, 100, 100, 100)
   sh.TextFrame.TextRange.Text = "hello there"
   End Sub
   

2. Create an add-in now. Create another deck, go to the VBE and put this in any module:

   Sub Auto_Open()
   Dim p As Presentation
   Set p = Presentations("Presentation3")
   Application.Run (p.Name & "!AddText")
   End Sub
   

3. Now save this add-in as a PowerPoint Add-in (.ppam). Install and load the addin (try unload/load again if it doesn't trigger) and you should have a textbox created on the active presentation.

Notice in #2 : Application.Run (p.Name & "!AddText"). The presenation name (including extention) and "!" with the sub routine's name are required to run a macro in another presentation.