Add greeting when recipients entered on a new mail - vba

Question

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?