One follow up to this:
In order to use DO FORM, the form must be an SCX, not a class.
If you prefer using form classes, you can do this instead.
Messageform is still a modal form, and still has a custom cRetVal property.
Instead of Releasing the form, Hide it
The Unload() event would no longer have any code.

** CallingForm.cmdAppend.Click()
LOCAL loMessageForm, lcValue
loMessageForm = NEWOBJECT("messageform", "messageform_lib")
** since it won't get returned, you'll have to grab it
lcValue = loMessageForm.cRetVal
IF NOT EMPTY(lcValue) AND VARTYPE(lcValue)="C"
  thisform.txtTarget.Value = lcValue
  ** do append code
ELSE
  ** some other cute messagebox :)
  ** don't do append
ENDIF


** messageform.cmdOK.Click()
thisform.cRetVal = ALLTRIM(thisform.txtMessage.Value)
thisform.Hide()

** messageform.cmdCancel.Click()
thisform.cRetVal = ALLTRIM(thisform.txtMessage.Value)
thisform.Hide()