There are some errors in your code. Here are some fixes.

The Form:

Option Explicit

Private Sub Command1_Click()
Dim oWinObj As New WindowObj
Dim oForm As Form

  Set oWinObj = New WindowObj
  Set oForm = oWinObj.OpenForm("Form2")
End Sub

Private Sub Command2_Click()
Dim oWinObj As New WindowObj

  Set oWinObj = New WindowObj
  oWinObj.OpenForm "Form2"
End Sub

The Class:

Option Explicit

Public Function OpenForm(sFormName As String) As Form
Dim oForm As Form

    Set oForm = Forms.Add(sFormName)
    oForm.Show
    
    Set OpenForm = oForm
End Function

Public Sub OpenFormSub(sFormName As String)
Dim oForm As Form

    Set oForm = Forms.Add(sFormName)
    oForm.Show
End Sub

>I want to open a form by passing the name of the form as a string
>to a function. I have:
>
>
>Public Sub LaunchForm(sFormName As String)
>
>  Dim oWinObj As New WindowObj
>  Dim oForm As Form
>
>  Set oWinObj = New WindowObj
>
>  oForm = oWinObj.OpenForm(sFormName)
>
>End Sub
>
>
>oWinObj is in a class:
>
>
>Public Function OpenForm(sFormName As String) As Form
>
>  Dim oForm As Form
>
>  Set oForm = Forms.Add(sFormName)
>  oForm.Show
>
>  OpenForm = oForm
>
>End Function
>
>
>Now it I don't try to return the instance of the form (oForm), it
>works fine. In other words, if the function above is defined as
>a Sub instead of a Function.
>
>I get the error 'Invalid use of property'
>
>Can someone help please?