Skatonni has this covered, and I didn't have time to post my version yesterday, but it's a little simpler to follow so I have added it to the thread. This version creates the message, but can easily be modified to work with an open message. The essence of either process is to employ the Word editor. Then it is as simple as editing a Word document using VBA.
Option Explicit
Sub AddHyperlink()
Dim olEmail As Outlook.MailItem
Dim olInsp As Outlook.Inspector
Dim wdDoc As Object
Dim oLink As Object
Dim oRng As Object
Dim strLink As String
Dim strLinkText As String
'The texts before and after the link
Const strText1 As String = "If you wish to download or view our latest catalogue, please simply follow this link: " & vbCr & vbCr
Const strText2 As String = vbCr & vbCr & "Should you wish to review or enquire about any of our products, please do not hesitate to get in touch."
strLink = "http://www.gmayor.com" ' the link address
strLinkText = "Click here for Graham Mayor's Web Site " ' the link display text
On Error Resume Next
Set olEmail = CreateItem(olMailItem)
With olEmail
.BodyFormat = olFormatHTML
Set olInsp = .GetInspector
Set wdDoc = olInsp.WordEditor
Set oRng = wdDoc.Range(0, 0)
oRng.Text = strText1
oRng.collapse 0
Set oLink = wdDoc.Hyperlinks.Add(Anchor:=oRng, _
Address:=strLink, _
SubAddress:="", _
ScreenTip:="", _
TextToDisplay:=strLinkText)
Set oRng = oLink.Range
oRng.collapse 0
oRng.Text = strText2
.Display
End With
lbl_Exit:
Exit Sub
End Sub