Frankly I wouldn't do it like that. Looking at your code it appears that you are intending to run the code from an application other than Outlook, so you need to ensure Outlook is started properly and for that you should use the code from the link at the top of the code below. Then you can use the Outlook Word editor to edit the message body directly and retain the signature associated with the account e.g. as follows:

Option Explicit

Sub outlookmail()
'Use the code from http://www.rondebruin.nl/win/s1/outlook/openclose.htm to start Outlook
Dim olApp As Object
Dim olEmail As Object
Dim olInsp As Object
Dim wdDoc As Object
Dim oRng As Object
Const olFormatPlain As Long = 1
Const olFormatHTML As Long = 2
Const olFormatRichText As Long = 3

    Set olApp = OutlookApp()
    Set olEmail = olApp.CreateItem(0)

    With olEmail
        .BodyFormat = olFormatRichText
        Set olInsp = .GetInspector
        Set wdDoc = olInsp.WordEditor
        Set oRng = wdDoc.Range
        oRng.Collapse 1
        oRng.Text = "Dear someone," & vbCr
        oRng.Collapse 0
        oRng.Select
        .Display
    End With
    
    Set olEmail = Nothing
    Set olInsp = Nothing
    Set wdDoc = Nothing
    Set oRng = Nothing
    Set olApp = Nothing
End Sub