Once way to do it would be to create a Macro that does this And set it up as a quick action button to create the e-mail instead of using the "New E-Mail" button.

The problem with the using the New E-Mail button is that this is programmed into Outlook and really no way to change the "funtion" of this. However, but creating a macro and assigning it a button, you can accomplish what you are asking.

Macro:

Public Sub CreateEmail()

'Create the oMail object
Dim oMail As Outlook.MailItem

'Create a new mail item
Set oMail = Application.CreateItem(olMailItem)

'Set the subject
With oMail
.Subject = "This is the subject"
.Display
End With

End Sub

Once you put this into a module and save it, do the following:

- File - Options
- Click Quick Access Toolbar
- In the dropdown box for "Choose commands from:", select "Macros"
- Click on the name of the Macro (Should be something like Project1.CreateEmail) and click "Add"
- Select the name from the "Customize Quick Access Toolbar" box and select modify
- You can rename itand even set an icon
- Click "Ok" until all the windows disappear
- The new Icon should appear in the top left corner. When you click on it, it should do what you want.

Some other notes:
You can also set the To, CC, BCC, Body, Send From, Reply To, and attachments as well by using .To, .CC, .BCC, .Body, .SentOnBehalfOfName, .ReplyRecipients.Add, and .Attachments.Add respectively.

Hope this helps!