Creating a VBA userform is easy enough - see attached, which you can unzip and import into the Outlook VBA editor. The bigger question is what you want to do with the form when you have displayed it.
You can call the attached form with the following code
Sub MyForm()
Dim olMsg As MailItem
Dim strSubject As String
Dim vWord As Variant
Dim i As Integer, j As Long
Dim bValid As Boolean
Dim oFrm As frmMessage
On Error Resume Next
bValid = False
Set olMsg = ActiveExplorer.Selection.item(1)
strSubject = olMsg.Subject
vWord = Split(strSubject, Chr(32))
For i = 0 To UBound(vWord)
If Len(vWord(i)) = 10 Then
bValid = True
Exit For
End If
Next i
If bValid = True Then
Set oFrm = New frmMessage
With oFrm
.txtAccount = vWord(i)
.txtSubject = strSubject
With .ListBox1
For j = 1 To 20
.AddItem Chr(64 + j)
Next j
End With
.Show
If .Tag = 1 Then MsgBox "Do something with the form"
End With
Unload oFrm
End If
lbl_Exit:
Set olMsg = Nothing
Set oFrm = Nothing
Exit Sub
End Sub