Dim WS As Worksheet

     Set WS = Sheets.Add(After:=Sheets(Worksheets.count))
     WS.name = txtSheetName.value

Dim WS As Worksheet
  MsgBox (WS Is Nothing)

    Set WS = Sheets.Add(After:=Sheets(Worksheets.count))
    '***** Nothing after this point gets processed *******
    MsgBox (WS Is Nothing)
    MsgBox WS.name

    WS.name = txtSheetName.value
    MsgBox WS.name

Dim WS as Worksheet
Set WS = Sheets.Add

You don't have to know where is it located, what's it's name, you just refer to it as WS.

If you still want to do this the "old fashioned" way, try this:

Sheets.Add.Name = "Test"

Dim WS As Worksheet
Dim i As Long

With ThisWorkbook
    Set WS = .Worksheets.Add(After:=.Sheets(.Sheets.Count))
End With

For i = 1 To 1000
    DoEvents
Next i

WS.Name = txtSheetName.Value