Add greeting when recipients entered on a new mail

I want to automatically create a greeting with the recipient's first name, on new messages.

For example, it should check the "To" field and if there is an email address, take the First Name from the contact and fill in the body of the message (ex. Dear [First Name],).

I don't know which method or event to use if I press New Message and which method or event to use to see if a recipient is added. (The macro should run every time a recepient is added.) If there are more then two recipients should be "Hello everyone,"

For a reply I am using the "Reply" event that occurs when the user selects the reply action.

Private Sub GMailItem_Reply(ByVal Response As Object, Cancel As Boolean)
    AutoAddGreetingToReply Response
End Sub

This finds the recipient name and adds a greeting to the reply message.

I also tried with a Word document that contains merge fields to mail merge but it doesn't work. Here is the code that I am using for email merge.

Option Explicit
Public Sub MailMergeAttachments()
    Dim Session As Outlook.NameSpace
    Dim currentExplorer As Explorer
    Dim Selection As Selection
    Dim oContact As ContactItem
    Dim oMail As MailItem
    Dim attach As Attachment
    Dim obj As Object
    Dim oWord As Word.Application
    Dim oDoc As Word.Document
    Dim tmp As String
    ' Uses current user's profile
    Dim enviro As String
    enviro = CStr(Environ("USERPROFILE"))

    ' Get Word
    Set oWord = GetObject(, "Word.Application")

Set oDoc = oWord.Documents(1)
 tmp = oDoc.FullName
 oDoc.Activate

oWord.Visible = True

    Set currentExplorer = Application.ActiveExplorer
    Set Selection = currentExplorer.Selection

  If Not TypeOf Selection.Item(1) Is Outlook.ContactItem Then
  MsgBox "You need to select Contacts first!"
  Exit Sub
  End If

For Each obj In Selection

'Test for ContactGroups
 If TypeName(obj) = "ContactItem" Then
 Set oContact = obj

Dim mText As String
Dim f As Word.Field

For Each f In oDoc.Fields

  If f.Type = wdFieldMergeField Then

 ' match Word mergefields with Outlook fields
      Select Case f.Code
       Case " MERGEFIELD First "
       mText = oContact.FirstName

       Case " MERGEFIELD Last "
       mText = oContact.LastName

       Case " MERGEFIELD Company "
       mText = oContact.CompanyName

       End Select

     f.Result.Text = mText
 End If
Next

Set oMail = Application.CreateItem(olMailItem)

With oMail
    .To = oContact.Email1Address
    .Subject = Left(oDoc.Name, Len(oDoc.Name) - 5)
    'The content of the document is used as the body for the email
    .Body = oDoc.Content
    .Attachments.Add enviro & "\Documents\instructions.pdf"
    .Display ' .send
End With

 End If
    Next
    Set oWord = Nothing
    Set Session = Nothing
    Set currentExplorer = Nothing
    Set obj = Nothing
    Set Selection = Nothing

End Sub

How do I run a macro when I select new message and how do I run it repeatedly when I enter new recipient/s and add personalised greeting in the body of the message?