I need a macro that adds a specific text, plus the content of my clipboard, at the end of the subject of all messages I have selected. How do I get the content of the clipboard automatically, without first pasting it into an Input Box?

Although Outlook VBA doesn't include a paste from clipboard function directly, you can use the MSForms dataobject to transfer the clipboard contents to a string which is then called from VBA. You could also use use the Word object model to copy the message body to the clipboard.

You can also use Word's 'Keep Source Formatting' to paste formatted text into an Item Body. Code sample is at Paste formatted text using VBA

Add code similar to this to your macro:

 Dim DataObj As MSForms.DataObject
 Set DataObj = New MSForms.DataObject
 DataObj.GetFromClipboard

strPaste = DataObj.GetText(1)

The finished code will look something like the following. Note, you will need to have a reference to the Forms library in Tools, References.

If you receive a "User-defined type not defined" you are missing the reference to Microsoft Forms 2.0 Object Library. If its not listed, add C:\Windows\System32\FM20.dll or C:\Windows\FM20.dll as a reference.

Sub AddtoSubject()
 Dim ex As Explorer
 Dim mail As MailItem
 Set ex = Application.ActiveExplorer
 Dim strPaste  As Variant
 
Dim DataObj As MSForms.DataObject
Set DataObj = New MSForms.DataObject
DataObj.GetFromClipboard

strPaste = DataObj.GetText(1)

If strPaste = False Then Exit Sub
If strPaste = "" Then Exit Sub

 For Each mail In ex.Selection
    mail.Subject = mail.Subject & " my text " & strPaste
    mail.Save
 Next mail
 
Set DataObj  = Nothing
End Sub

Copy to Clipboard

What about going in the other direction: copying text to the clipboard? Use PutInClipboard to capture the text.

Remember, if you receive a "User-defined type not defined" you are missing the reference to Microsoft Forms 2.0 Object Library. If its not listed, add C:\Windows\System32\FM20.dll or C:\Windows\FM20.dll as a reference.

Sub CapturetoClipbaord()
Dim oMail As MailItem
DataObj As MSForms.DataObject

    Set oMail = ActiveExplorer().Selection.Item(1)
    Set DataObj = New MSForms.DataObject
    DataObj.SetText oMail.Body
    DataObj.PutInClipboard
End Sub

Use Word Object Model to Copy (and Paste)

Current versions of Outlook use Word as the email as the email editor and can use the Word object model library to do things not normally supported in Outlook.

This sample copies the body of the selected message to the clipboard. To paste, use
objSel.PasteAndFormat (wdFormatOriginalFormatting)

Don't forget to set a reference to Word's Object model in Tools, References.

Sub CopyMessage()
    Dim objMail As Outlook.MailItem
    Dim objInsp As Inspector
    Dim objDoc As Word.Document
    Dim objSel As Word.Selection

Set objMail = Application.ActiveExplorer.Selection.Item(1)

  If Not objMail Is Nothing Then
        If objMail.Class = olMail Then

            Set objInsp = objMail.GetInspector
            If objInsp.EditorType = olEditorWord Then
                Set objDoc = objInsp.WordEditor
                Set objWord = objDoc.Application
                Set objSel = objWord.Selection
        With objSel
            .WholeStory
            .Copy
       End With
            End If
        End If
    End If
    
    Set objMail = Nothing
End Sub

This sample shows how to paste the copied content into a new message:

    Dim objMail As Outlook.MailItem
    Dim objInsp As Inspector
    Dim objDoc As Word.Document
    Dim objSel As Word.Selection

Set objMail = Application.CreateItem(olMailItem)
With objMail
  .To = "Alias@domain.com"
  .Subject = "This is the subject"
  .Display
End With

Set objInsp = objMail.GetInspector
If objInsp.EditorType = olEditorWord Then
    Set objDoc = objInsp.WordEditor
    Set objWord = objDoc.Application
    Set objSel = objWord.Selection
        
objSel.PasteAndFormat (wdFormatOriginalFormatting)
End If

Set objMail = Nothing
End Sub

Paste format types are below. For more information see WdRecoveryType Enumeration (Word).